PIEBALD  RATS  AND  SELECTION 


AN  EXPERIMENTAL  TEST  OF  THE  EFFECTIVENESS  OF 

SELECTION  AND  OF  THE  THEORY  OF  GA^IETIC 

PURITY  IN  MENDELIAN  CROSSES 


QH\Ay 


'?3N5 


BY 
W.  E.  CASTLE  AND  JOHN  C.  PHILLIPS 


0H431 
C37 


WASHINGTON,  D.  G. 
Published  by  the  Caknegie  Institution  of  Washington 

1914 


SH^p  S.  M.  'Ml  IGtbrarg 


North  (Earolina  ^tate  HniuprHitg 

QH431 
C37 


Nortri^rolin^tat^ibrary 

PIEBALD  RATS  AiND  SELECTION 


AN  EXPERIMENTAL  TEST  OF  THE  EFFECTIVENESS  OF 

SELECTION  AND  OF  THE  THEORY  OF  GAMETIC 

PURITY  IN  MENDELIAN  CROSSES 


BY 
W.  E.  CASTLE  AND  JOHN  C.  PHILLIPS 


WASHINGTON,  D 
Published  by  the  Carnegie  Institut 

1914 


D.  H.  HILL  LIBRARY 
lie.  STrtiE  UNIVERSITY 


THIS  BOOK  IS  DUE  ON  THE  DATE 
INDICATED  BELOW  AND  IS  SUB- 
JECT TO  AN  OVERDUE  FINE  AS 
POSTED  AT  THE  CIRCULATION 
DESK. 


lOOM/5-79 


j0m*'m  'MT^^vV^' 


Carnegie  Institution  of  Washington,  Publication  Xo.  195 


Paper  No.  21  of  the  Station  for  Experimental  Evolution 
AT  Cold  Spring  Harbor,  New  York 

From  the  Laboratory  of  Genetics 
of  the  Bussey  Institution 


■'  t  ?'$  Book 
issued 

^E8  181914 


PRESS  OF  GIBSON  BROTH KRS.  INC. 
WASHINGTON,  D.  C. 


CONTENTS. 


Page. 

Introduction .5 

Material  and  methods 7 

Plus  selection  series 9 

Minus  selection  series 12 

Return  selection 13 

Crosses  with  wild  rats IG 

Crosses  with  black  "Irish"  rats 18 

Plus  selection  of  "extracted  hooded"  rats 20 

Crosses  of  the  plus  race  with  the  minus  race 22 

Summary  of  results 22 

Discussion 23 

The  "mutant"  series 25 

Bibliography 31 

Tables 32-54 

Explanation  of  plates 56 


PIEBALD  RATS  AND  SELECTION. 


INTRODUCTION. 

The  fundamental  importance  of  Mendel's  law  of  heredity  is  generally 
recognized  among  biologists.  It  is  a  working  hypothesis  whose  utility 
is  fully  substantiated  by  abundant  results  daily  increasing  in  amount. 
But  biologists  are  not  in  agreement  as  to  how  much  this  law  includes. 
All  perhaps  would  agree  that  it  implies  the  existence  in  the  germ  cell  of 
specific  determiners  essential  for  the  production  of  particular  character- 
istics in  the  offspring.  Further,  no  one  probably  will  object  to  the 
statement  that  it  implies  a  dual  or  duplex  condition  of  the  zygote  as 
regards  determiners  and  a  simple  or  simplex  condition  of  the  gamete. 
Thirdly,  the  fact  will  be  admitted  by  all  that  most  mendelizing  char- 
acters are  wholly  independent  of  each  other  in  heredity,  for  which 
reason  we  are  forced  to  suppose  that  their  determiners  are  distinct 
within  the  germ-cell. 

But  beyond  these  few  generalizations  great  diversity  of  opinion 
exists.  As  regards  the  very  nature  and  function  of  the  determiners, 
some  consider  them  unvarying,  and  explain  the  observed  variation  of 
mendelizing  characters  in  organisms  as  due  to  a  modifying  action  of 
other  determiners.  At  one  time  even  a  modifying  action  of  other 
determiners  was  denied,  and  the  theory  was  advanced  that  the  gametes 
extracted  from  a  mendelian  cross  are  pure  as  regards  the  single  char- 
acters which  may  have  been  concerned  in  that  cross.  Investigations 
carried  out  by  Castle  have  done  something  to  dispel  this  idea.  In 
particular  it  was  shown  (Castle,  1905,  1906;  Castle  and  Forbes,  1906) 
that  in  guinea-pigs,  polydactylism,  long-hair,  and  rough  coat  are  men- 
delizing characters  which  are  affected  in  the  degree  of  their  develop- 
ment by  crosses — that  is,  when  these  characters  are  "extracted"  from 
crosses  the  characters  are  not  exactly  the  same  as  before;  hence  the 
gametes  are  not  ''pure." 

The  experimental  result  is  not  denied,  but  in  order  to  save  the  sub- 
stance of  the  theory  its  advocates  now  suppose  that  the  determiners 
have  not  changed,  but  in  consequence  of  the  cross  certain  modifiers 
have  become  associated  with  them  which  change  their  appearance  in 
the  organism.  The  real  unchanging  thing  is  now  called  the  "geno- 
type," its  appearance  the  "phenotype. " 

In  this  genotype  theory  we  are  dealing  only  with  a  new  and  more 
refined  aspect  of  the  "theory  of  pure  gametes."  It  is  not  a  necessary 
part  of  mendelism,  not  even  an  original  part;  but  it  is  ver}^  important 
for  us  to  know  whether  it  is  true  or  not.  For  if  it  is  true,  selection 
unattended  by  hybridization  is  largely  a  waste  of  time,  as  De  Vries  and 
Johannsen  have  maintained,  and  Jennings  and  Pearl  have  reiterated. 

5 


6  INTRODUCTION. 

The  investigation  which  we  are  about  to  describe  was  started  six 
years  ago  to  test  the  vahdity  of  the  theory  of  pure  gametes  which  was 
then  current.  Pure  "genes"  had  not  yet  been  invented.  The  inves- 
tigation has  been  in  continuous  progress  ever  since,  and  while  we  expect 
to  continue  it  further,  it  seems  to  us  desirable  that  the  results  already 
obtained  be  presented  for  criticism. 

Some  conception  of  the  work  entailed  in  the  investigation  may  be 
gathered  from  the  statement  that  we  have  during  its  progress  reared 
and  studied  the  color  pattern  of  over  25,000  rats.  A  long  and  arduous 
investigation  of  this  kind  has  been  made  possible  by  a  series  of  grants 
from  the  Carnegie  Institution  of  Washington  made  to  the  senior  author, 
for  which  he  here  makes  grateful  acknowledgment.  Thanks  are  also 
due  to  Dean  W.  C.  Sabine,  of  Harvard  University,  for  encouraging  and 
supporting  the  work  in  a  variety  of  ways. 


MATERIAL    AND    METHODS. 


MATERIAL  AND  METHODS. 

In  June  1906  Dr.  Hansford  MacCurdy  completed,  under  the  direc- 
tion of  the  senior  author,  a  study  of  the  inheritance  of  color  in  rats. 
His  studies  had  shown  that  the  piebald  pattern  of  "hooded"  rats 
behaves  as  a  mendelian  recessive  character  in  relation  to  the  uniform 
or  nearly  uniform  coloration  of  wild  rats,  but  that  the  hooded  pattern, 
when  extracted  from  a  cross  with  wild  stock,  shows  a  different  vari- 
ability, the  pigmentation  of  the  extracted  recessives  being  increased  in 
extent.  This  result  was  interpreted  as  showing  the  unsoundness  of  the 
current  doctrine  of  ''purity  of  the  gametes"  in  mendelian  crosses. 

Upon  the  conclusion  of  Dr.  MacCurdy's  experiments,  the  pedigreed 
stock  which  he  had  used  was  not  entirely  discarded.  A  certain  portion 
of  it  was  utilized  for  new  experiments  designed  to  show  whether  the 
"hooded"  coat-pattern  can  be  modified  by  selection  unattended  by 
cross-breeding. 

Two  series  of  selections  were  started  in  October  1907,  in  one  of  which 
animals  w^re  chosen  as  parents  which  had  pigmentation  as  extensive  as 
possible.  This  we  may  call  the  pZws  series.  In  the  other  series  animals 
were  chosen  as  parents  which  had  pigmentation  as  restricted  as  pos- 
sible.    This  we  may  call  the  minus  series. 

During  the  academic  year  1906-7,  the  experiments  were  in  immediate 
charge  of  Mr.  W.  G.  Vinal;  during  1907-8  the  plus  series  was  in  charge 
of  Mr.  H.  S.  Rand,  while  the  minus  series  was  in  charge  of  Mr.  F.  C. 
Bradford.  Throughout  this  time  the  experiments  were  closely  super- 
vised by  the  senior  author,  w^ho  assisted  in  the  "grading  "  of  every  litter 
of  young.  In  October  1908  the  junior  author  began  his  association  in 
the  experiments,  which  has  continued  up  to  the  present  time.  Through- 
out these  five  j^ears  he  has  looked  after  the  details  of  the  experiments 
almost  continuously,  but  both  authors  have  in  most  cases  taken  part 
together  in  the  grading  of  the  young,  and  in  no  case  has  the  grading 
been  done  except  under  the  immediate  supervision  of  one  or  the  other 
of  the  authors.  This  fact  is  stated  to  show  that  the  personal  element 
in  the  grading  has  been  kept  as  constant  as  possible.  In  the  tabulation 
of  results  and  computation  of  statistical  constants,  the  authors  have 
worked  together.  This  statement  of  results  is  written  by  the  senior 
author. 

During  the  year  1906-7  the  young  rats  were  graded  by  the  method 
used  by  MacCurdy  and  Castle  (1907)  that  is,  the  back-stripe  was 
measured  and  a  calculation  made  of  the  percentage  of  the  dorsal  sur- 
face posterior  to  the  hood  which  was  pigmented.  But  on  account  of  the 
irregular  outline  of  the  back-stripe  in  many  individuals  the  method  of 
measurement  was  found  to  be  at  best  a  rough  one,  as  well  as  extremely 
laborious.     Accordingly  in  the  summer  of  1907  a  set  of  arbitrary  grades 


8  PIEBALD    RATS   AND    SELECTION. 

was  adopted,  which  is  shown  at  the  top  of  Plate  1 .  Each  j^ouiig  rat  was 
classed  in  that  grade  which  it  most  nearly  approached  in  amount  of 
pigmentation.  Skins  of  rats  graded  from  —  3^  to  +4f  are  shown  in 
the  middle  and  lower  rows  of  Plate  1.  The  grading  was  done  when 
the  rats  were  about  three  or  four  weeks  old,  at  which  time  selected 
individuals  were  reserved  as  the  parents  for  a  later  generation,  the 
remainder  being  discarded.  This  method  has  been  followed  ever 
since  its  adoption  and  the  data  thus  obtained  are  summarized  in  the 
tables,  which  cover  the  breeding  operations  of  a  little  more  than  six 
years,  1907-1913. 

The  grouping  of  the  young  in  a  series  of  generations  is  only  approx- 
imately accurate,  for  practical  considerations  have  often  led  us  to  mate 
together  animals  which  belonged  to  different  generations  of  offspring. 
When,  for  example,  an  animal  of  generation  2  was  mated  with  one  of 
generation  4,  the  question  would  arise:  To  what  generation  do  the  off- 
spring belong?  In  deciding  this  question  we  simply  added  one  to  the 
mean  of  the  generations  to  which  the  respective  parents  belonged.     In 

2  -)-  4 

the  foregoing  case  this  would  be  — - — 1-1  =  4. 

In  case  one  parent  belonged  to  generation  2  and  the  other  to  generation 

2  -f-  3 

3,  a  fractional  result  would  be  obtained,  thus — - — 1-1=35.     In  making 

up  the  summaries  of  the  generations  as  given  in  the  tables,  offspring 
like  the  foregoing,  of  generation  3§,  were  divided  equally  between  gener- 
ations 3  and  4,  alternate  litters  of  young  as  recorded  in  the  ledger  being 
assigned  to  each.  Offspring  belonging  to  generations  2f  and  Z\  were 
tabulated  in  generation  3;  those  belonging  to  generations  3f  and  43: 
were  tabulated  in  generation  4,  etc.  While,  therefore,  the  genera- 
tions as  tabulated  overlap,  it  is  clear  that  they  include  groups  of  off- 
sprmg  of  selected  parents  each  the  result  of  one  additional  selection  over 
the  preceding  group. 

The  early  generations  include  too  few  individuals  to  be  of  much 
statistical  value,  but  where  the  number  of  offspring  rises  to  500  or  over, 
the  statistical  constants  acquire  undoubted  value.  The  datahavebeen 
given  in  the  form  of  correlation  tables  which  will  repay  careful  study. 
In  the  tables  a  single  entry  has  been  made  for  each  individual  offspring 
in  that  row  which  corresponds  with  the  mean  grade  of  its  two  parents. 
Thus,  if  one  parent  were  of  grade  2  and  the  other  of  grade  2|,  the  off- 
spring would  be  entered  in  the  row  21  along  with  the  offspring  of  parents 
both  of  grade  2j.  Offspring  of  parents  whose  mean  grade  fell  betwee?i 
the  rows  given  in  the  tables  were  divided  equally  between  the  adjacent 
rows,  alternate  litters  being  assigned  to  each.  Thus,  if  the  mean  grade 
of  the  parents  were  21^,,  alternate  litters  of  offspring  would  be  entered  in 
row  2  and  in  row  2|. 


PLUS    SELECTION    SERIES.  9 

PLUS  SELECTION  SERIES. 

This  series  begins  with  pairs  ranging  in  average  grade  from  +1-87 
to  +3.  From  these  parents  were  obtained  150  young,  which  range  in 
grade  from  + 1  to  +3,  as  is  shown  in  Table  1.  It  will  be  observed  that 
the  lower-grade  parents  have  on  the  average  lower-grade  offspring  than 
the  higher-grade  parents.  But  in  no  case  is  the  average  grade  of  the 
offspring  as  great  as  that  of  their  parents.  Thus  L87  parents  had  1.82 
offspring  (average  grade) ;  2.00  parents  had  1.76  offspring;  2.25  parents 
had  1.87  offspring;  and  so  on  to  3.00  parents,  which  had  2.35  offspring. 
There  is  a  falling  back  in  grade  or  "regression"  of  the  offspring  as  com- 
pared with  their  parents,  which  increases  in  amount  as  the  grade  of  the 
parents  becomes  higher.  (See  column  "Regression"  in  Table  1 .)  The 
parents  of  this  first  generation  were  chosen  because  of  their  high  grade. 
They  were  all  probably  in  grade  above  the  general  average  of  the  popu- 
lation from  which  they  were  selected.  In  the  case  of  those  which 
deviate  most  from  the  general  average  the  regression  is  greatest,  as  we 
should  expect. 

This  phenomenon  of  regression,  which  is  a  very  general  one  in  cases 
of  selection,  was  first  observed  by  Galton  in  selecting  sweet-peas  of 
varying  size  from  a  mixed  population.  Later  Johannsen,  who  repeated 
the  experiment  with  beans,  found  that  by  pedigree  culture  he  was  able 
to  break  the  mixed  population  up  into  pure  lines  within  which,  con- 
sidered singly,  no  regression  occurred.  We  shall  need  later  to  return 
to  this  subject  and  consider  whether  pure  lines  free  from  regression 
exist  or  can  be  produced  as  regards  the  hooded  pattern  of  rats. 

Returning  to  the  examination  of  Table  1,  since  the  high-grade  parents 
produce  higher-grade  offspring  than  do  the  low-grade  parents,  it  is 
evident  that  we  might  hope  by  further  selection  either  to  isolate  a  pure 
line  of  high-grade  rats  which  would  be  free  from  regression  and  therefore 
stable,  or  else  to  advance  the  grade  of  the  offspring  still  higher,  even 
though  regression  persists.  As  a  measure  of  the  extent  to  which  high- 
grade  parents  have  high-grade  offspring  and  vice  versa,  in  each  genera- 
tion, we  may  employ  the  well-known  correlation  coefficient.  This  for 
Table  1  is  0.30. 

The  second  generation  in  the  plus  series  (Table  2)  includes  the  off- 
spring of  parents  which  appear  as  offspring  of  the  higher  grades  in 
Table  1,  together  with  a  few  individuals  which  appear  in  Table  2  both 
as  offspring  and  as  parents  of  other  offspring,  by  reason  of  their  having 
been  mated  with  generation  1  individuals  and  so  having  produced 
generation  1|  offspring,  as  explained  on  page  8.  To  obtain  larger 
numbers  of  offspring,  several  new  pairs  were  added  to  the  experiment 
in  this  generation,  which  do  not  appear  in  Table  1  either  as  offspring  or 
as  parents,  but  which  were  derived  from  the  same  general  stock  as  the 
parents  of  generation  1.     Their  inclusion  here  accounts  for  the  very 


10  PIEBALD    RATS   AND    SELECTION. 

low  range  of  the  offspring  in  Table  2,  which  extends  from  —1.00  to 
+  3.75.  The  parents'  range  (means  of  pairs)  extends  from  2.00  to  3.12. 
The  grand  average  of  the  parents  is  2.52,  that  of  the  offspring  is  1.92. 
The  correlation  between  grade  of  parents  and  grade  of  offspring  is  0.32. 

From  this  point  on  in  the  series  no  new  stock  was  added  and  each 
generation  of  offspring  furnished  the  parents  for  the  following  genera- 
tion, except  for  the  slight  overlapping  of  generations  when  parents  of 
different  generations  were  mated  with  each  other,  as  has  already  been 
explained. 

In  generation  3,  Table  3,  the  parents  ranged  from  2.12  to  3.37  in 
grade,  the  ofTspring  from  0.75  to  4.00.  The  mean  of  the  parents  was 
2.73,  that  of  the  offspring  2.51.  The  degree  of  correlation  between 
parents  and  ofTspring  is  expressed  by  the  coefficient  0.33  (a  perfect 
correlation  would  give  1.00). 

In  generation  4,  Table  4,  the  selection  of  parents  became  considerably 
more  rigid ;  most  of  the  parental  pairs  were  of  grade  3  or  higher,  their 
average  being  3.09.  The  average  grade  of  the  offspring  was  2.73,  their 
range  extending  from  0.75  to  3.75.  The  correlation  in  this  generation 
fell  very  low,  to  0.07,  not  because  of  a  lessened  regression  but  rather 
because  of  a  very  high  regression  on  the  part  of  the  offspring  of  high- 
grade  parents. 

In  generation  5,  Table  5,  the  grade  of  the  selected  parents  ranged 
from  2.75  to  4.12,  its  mean  being  3.33.  The  offspring,  showing  the 
usual  regression,  ranged  from  0.75  to  4.25,  their  mean  grade  being  2.90. 
The  correlation  between  parents  and  offspring  in  this  generation  was 
0.16.  The  number  of  individuals  comprising  this  generation  of  off- 
spring was  610. 

It  is  scarcely  necessary  to  discuss  separately  the  correlation  table 
for  each  of  the  next  eight  generations,  Tables  6  to  13.  The  number  of 
offspring  rises  to  a  maximum  (1,408)  in  generation  8,  Table  8;  then 
declines  to  less  than  200  in  generation  13.  But  as  this  generation  and 
the  preceding  one  are  still  iDeing  produced,  it  is  probable  that  the  num- 
ber recorded  will  be  considerably  increased  before  the  generation  is 
complete.  The  means  of  parents  and  offspring  and  the  other  statistical 
constants  for  the  several  generations  can  be  most  easily  compared  by 
reference  to  Table  14.  Leaving  out  of  consideration  the  exceptional 
generation,  2,  the  following  will  be  observed: 

(1)  The  mean  of  the  selected  parents  has  steadily  advanced  through- 
out the  series,  as  has  also  that  of  their  offspring. 

(2)  The  variability  (standard  deviation)  of  the  parents  as  a  group 
has  decreased  somewhat  as  increase  in  numbers  made  a  more  rigid  selec- 
tion possible;  that  of  the  offspring  has  undergone  a  similar  change. 

(3)  The  correlation  between  parents  and  offspring  has  not  materially 
changed.  The  average  of  the  correlation  coefficients  for  the  entire 
series  is  0.194,  for  the  last  three  generations  it  is  0.175,  for  the  three  pre- 


PLUS    SELECTION    SERIES. 


11 


ceding  generations  it  is  0.141,  for  the  three  which  precede  those  it  is 
0.185,  while  for  the  first  four  generations  it  is  0.253.  In  every  case  the 
correlation  is  positive— that  is,  the  higher-grade  parents  have  higher- 
grade  offspring  and  vice  versa. 

(4)  The  offspring  as  a  group  average  lower  in  grade  than  their 
parents — that  is,  their  mean  regresses  on  that  of  the  selected  parents, 
but  because  of  the  higher  mode  about  which  variation  occurs  in  each 
generation  certain  of  the  offspring  are  of  higher  grade  than  their  parents. 
Thus  an  elevation  of  the  grade  of  the  parents  in  the  next  generation  is 
made  possible. 

(5)  With  the  selection  of  more  extreme  parents,  the  absolute  regres- 
sion of  the  offspring  has  not  increased,  but  on  the  contrary  has  slightly 
diminished — that  is,  the  advance  made  by  the  parents  is  retained  by 
their  offspring. 

In  Table  15  have  been  brought  together  for  comparison  the  means  of 
the  several  horizontal  rows  of  Tables  1  to  13.  By  examining  the  vertical 
columns  of  Table  15  the  mean  grade  of  the  offspring  of  parents  of  a 
particular  grade  in  any  generation  may  be  compared  at  a  glance  with 
that  of  parents  of  the  same  grade  in  any  other  generation.  By  running 
the  eye  dow^n  the  columns,  it  will  be  observed  that  the  mean  grade  of 
the  offspring  tends  to  increase  upon  repeated  selection.  Thus  parents 
of  grade  3.75  appear  first  in  generation  4,  the  grade  of  their  offspring 
being  2.75;  the  offspring  of  such  parents  in  subsequent  generations 
grade  in  order,  3.07,  3.22,  3.35,  3.49,  3.50,  3.69,  3.75,  and  3.83  (twelfth 
generation  not  complete) .  The  difference  between  parents  and  offspring 
in  this  series  grows  less  and  less  and  finally  disappears  altogether.  If 
the  grade  of  3.75  parents  in  this  series  is  compared  with  the  grade  of  all 
offspring  in  the  corresponding  generations  we  have  the  following: 

Table  A. 


Genera- 
tion. 

Mean  of 
offspring  of 
3.75  parents. 

Mean  of 
offspring  of 
all  parents. 

Genera- 
tion. 

Mean  of  offspring 
of  3.75  parents. 

Mean  of  offspring 
of  all  parents. 

4 
5 
6 

7 
8 

2.75 
3.07 
3.22 
3.35 
3.49 

2.73 
2.90 

3,n 

3.20 
3.48 

9 
10 
11 
12 

3.50 
3.69 
3.75 
3.83  (35 individuals) 

3.54 

3.73 
3.77 
3.94  (590 indiWduals) 

In  generation  4  the  3.75  parents  represented  the  most  advanced  indi- 
viduals of  the  series,  a  whole  grade  in  advance  of  the  general  average 
of  the  race.  Their  offspring  showed  a  correspondingly  large  regression. 
The  general  average  of  the  race  steadily  advanced  in  later  generations 
until  in  generation  11  it  equaled  that  of  the  3.75  parents;  then  the 
regression  vanished.  In  the  following  generation,  12  (which  is  still 
incomplete,  but  in  which  the  average  of  the  offspring  thus  far  is  3.94), 
the  3.75  group  of  parents,  which  are  now  below  the  average  of  the  race, 


12  PIEBALD    RATS   AND    SELECTION. 

actually  produce  offspring  of  higher  grade  than  themselves,  viz,  3.83. 
It  will  thus  be  seen  that  the  regression  is  uniformly  toward  the  mean  of  the 
race  and  changes  its  direction  ivhen  that  mean  changes  its  position  with 
reference  to  a  particular  grade  of  parents.  This  conclusion  is  supported 
by  other  columns  of  Table  15,  but  is  best  illustrated  by  this  particular 
case  because  here  the  selection  has  extended  over  a  greater  number  of 
generations  than  elsewhere  in  the  series. 

If  one  examines  the  horizontal  rows  of  Table  15,  he  finds  in  general 
that  numbers  increase  toward  the  right.  Exceptions  are  coimiionest 
toward  the  ends  of  the  rows  Vv^iere  fewest  individuals  are  represented. 
This  increase  means  that,  within  any  generation,  as  the  grade  of  the 
parents  rises,  that  of  their  offspring  rises  also.  Since  in  general  the 
selected  parents  are  above  the  general  average  of  the  race  for  the  time 
being,  regression  is  naturally  downward  in  nearly  all  cases. 

From  what  precedes  we  may  conclude  (1)  that  in  this  series  of  rats 
the  somatic  character  (appearance)  of  an  individual  is  in  general  a  true 
indication  of  its  germinal  character,  since  the  higher  the  grade  of  the 
parents  the  higher  the  grade  of  the  offspring,  and  vice  versa;  but  that  (2) 
the  somatic  character  of  an  individual  is  not  a  j)erfect  index  of  its  ger- 
minal character,  since  the  offspring  of  aberrant  individuals  are  less 
aberrant  than  themselves,  i.  e.,  the  offspring  regress  toward  the  mean  of 
the  race;  yet  that  (3)  by  selection  of  plus  variations  we  can  displace,  in  a 
plus  direction,  not  only  the  mean  of  the  race,  but  also  the  upper  and 
lower  limits  of  its  variation,  the  total  amount  of  variability  (standard 
deviation)  being  thereby  only  slightly  decreased. 

MINUS  SELECTION  SERIES. 

This  series  begins  with  selected  parents  ranging  in  grade  from  — 1.25 
to  — 1.87.  Their  average,  if  each  pair  is  weighted  in  proportion  to  the 
number  of  its  offspring,  is  —1.46.  The  offspring  (Table  16),  like  the 
offspring  of  the  original  plus  selections,  regress  toward  grade  0.  They 
range  in  grade  from  +0.25  to  —2.00,  their  mean  being  —1.00.  The 
total  number  of  offspring  recorded  in  this  generation  is  only  55,  this 
being  too  small  to  warrant  the  calculation  of  a  correlation  coefficient. 

Generation  2  (Table  17)  is  somewhat  larger,  but  still  too  small  to 
make  statistical  constants  based  upon  it  of  much  consequence.  The 
offspring  show  substantially  the  same  range  of  variation  as  in  the  pre- 
vious generation,  but  with  a  slightly  higher  average  (  —  1.07).  The 
coefficient  of  correlation  (  —  0.03)  is  negative,  but  too  small  to  be  signifi- 
cant. The  record  of  the  next  eleven  generations  will  be  found  summar- 
ized in  Tables  18  to  28,  or  in  more  condensed  form  in  Tables  29  and  30. 
Generation  13  (Table  28)  is  still  incomplete. 

The  mean  of  the  parents  steadily  rises  from  — 1.56  in  generation  3  to 
—  2.50  in  generation  13.  The  mean  of  the  offspring  rises  by  like  incre- 
ments from  —  1 .  18  in  generation  3  to  —  2.39  in  generation  13.     There  is 


MINUS    SELECTION    SERIES.  13 

throughout  these  generations  a  positive  correlation  bet^veen  parents 
and  offspring.  This  amounts  on  the  average  to  0. 137  as  compared  with 
0.193  observed  in  the  phis  selection  series.  The  absolute  change  in 
amount  of  pigmentation  is  no  doubt  less  in  the  minus  selection  than  in 
the  plus  selection  series,  but  if  the  change  were  recorded  as  percentage 
decrease  of  pigmentation  in  one  case  and  percentage  increase  in  the  other, 
the  change  indicated  would  probably  be  as  great  in  one  as  in  the  other. 
In  the  minus  as  in  the  plus  series  we  observe : 

(1)  The  character  of  the  offspring  varies  with  that  of  the  parents; 
high-grade  parents  have  high-grade  offspring  and  vice  versa. 

(2)  The  variability  of  the  race  (as  indicated  b}^  the  standard  devia- 
tion) undergoes  some  reduction  and  the  limits  of  variation,  both  upper 
and  lower,  are  displaced  in  the  direction  of  the  selection. 

(3)  The  regression  from  a  new  and  extreme  class  of  parents  is  at  first 
large,  but  decreases  as  the  selection  is  repeated  and  finally  disappears 
altogether  when  the  average  of  the  race  becomes  equal  to  the  particular 
grade  under  discussion. 

RETURN  SELECTION. 

The  plus  and  minus  selection  series  already  described  make  it  clear 
that  one  can,  in  a  race  of  hooded  rats,  either  increase  or  decrease  the 
average  pigmentation  at  will,  and  at  the  same  time  secure  more  advanced 
stages  either  of  pigmentation  or  of  depigmentation  than  those  pre- 
viously occurring  in  the  race.  The  question  now  arises,  are  these 
changes  permanent;  will  these  displaced  means  retain  their  new  posi- 
tion, if  the  race  is  left  to  itself;  or  will  the  newly  obtained  stages  vanish 
as  soon  as  selection  is  suspended?  A  presumption  that  the  changes 
will  prove  permanent  is  afforded  by  the  gradual  decrease  of  regression 
and  its  final  reversal  in  the  case  of  offspring  of  a  particular  grade,  upon 
repeated  selection  made  in  the  same  direction.  (See  page  12.)  But  in 
order  to  test  the  matter  more  directly  and  thoroughly,  the  experiment 
has  been  repeatedly  made  of  reversing  the  course  of  selection,  after  it  had 
been  in  progress  for  several  generations,  with  a  view  of  ascertaining 
whether  the  return  toward  the  former  condition  of  the  race  would  be 
made  more  speedily  and  easily  than  the  original  departure  from  it  had 
been. 

The  first  experiment  of  this  sort  was  a  return  selection  from  genera- 
tion 6  (and  6|)  of  the  minus  selection  series.  The  parents  of  generation 
6  (Table  21)  averaged  —1.86  in  grade;  the  average  grade  of  their  off- 
spring w^as  —1.56,  a  regression  of  0.30.  The  range  of  the  offspring 
extended  from  0  to  —2.50.  Some  low-grade  offspring  were  chosen  foi- 
a  return  selection  series  (Table  31).  The  mean  grade  of  the  selected 
pairs  ranged  from  —0.37  to  —0.87,  their  mean  being  —0.60.  These 
parents  produced  118  offspring,  whose  average  grade  was  —1.28,  a 
regression  of  0.68  in  a  direction  contrary  to  that  of  the  regression  in  the 


14  PIEBALD    RATS   AND    SELECTION. 

miJius  selection  series.  The  large  amount  of  the  regression  might  seem 
to  imply  that  it  was  even  more  difficult  to  return  toward  the  former 
state  of  the  race  (in  the  neighborhood  of  0)  than  it  had  been  to  depart 
from  it,  but  this  can  not  be  insisted  on,  because  the  number  of  indi- 
viduals under  observation  is  not  sufficiently  large.  To  test  the  reality 
and  permanenc}^  of  the  reversed  regression,  the  selection  was  repeated 
five  additional  times,  altogether  six  successive  return  selections  being 
made  with  the  idea  of  undoing  what  had  been  effected  by  six  original 
selections  in  an  opposite  direction.  The  result  of  the  second  successive 
return  selection  is  shown  in  Table  32.     The  parents  here  were  of  grade 

—  0.50  and  they  produced  19  offspring  of  the  average  grade  —0.95,  a 
regression  of  0.45  away  from  0  as  before. 

Table  33  shows  the  result  of  the  third  return  selection.  Individuals 
entered  in  Table  32  as  offspring  appear  here  as  parents.  Only  those 
pairs  which  were  of  mean  grade,  —0.25  or  —0.37,  should  really  be 
regarded  as  a  third  return  selection.  They  gave  offspring  with  mean 
grades  of  —0.63  and  —0.86  respectively,  which  show  regression  of  0.38 
and  0.49  aumy  from  0. 

But  Table  33  shows  also  the  character  of  3'oung  produced  by  —1.12 
and  —1.25  parents  in  this  same  third  return-selection  generation,  i.  e., 
by  unselected  parents  of  the  generation  in  question.  Their  young  also 
regress  aivay  from  0 — that  is,  in  the  direction  of  the  original  selection. 
The  — 1.12  parents  produced  — 1.61  offspring,  a  regression  of  0.49,  while 
the  — 1.25  parents  produced  — 1.35  offspring,  a  regression  of  0.10.  For 
Table  33  as  a  whole  the  regression  au'a?/ /row  0  averages  0.31. 

A  fourth  generation  in  the  return-selection  series  is  summarized  in 
Table  34.  The  parents  are  of  mean  grade  —0.63;  their  50  offspring  are 
of  mean  grade  —1.17,  a  regression  amounting  to  0.54  away  from  0  and 
in  the  direction  of  the  six  generations  of  original  selection. 

Table  35  contains  the  results  of  the  fifth  generation  of  the  series. 
The  parents  are  here  of  mean  grade  —0.65.  The  number  of  offspring 
is  very  small  (13),  but  they  nevertheless  show  the  reversed  regression 
which  characterized  the  four  preceding  generations.     Their  mean  was 

—  0.75,  a  regression  of  0.10  away  from  0. 

A  sixth  and  final  generation  in  this  return-selection  experiment  is 
summarized  in  Table  36.  It  includes  36  offspring  of  mean  grade  —  0.39, 
the  mean  of  the  parents  being  —0.26,  a  regression  of  0.13  away  from  0. 
It  will  be  seen,  therefore,  that  the  effect  of  the  six  original  selections  had 
not  been  entirely  overcome  by  an  equal  number  of  return  selections. 
The  reason  for  this  is  obvious.  Much  smaller  numbers  are  concerned  in 
the  return  selections  than  in  the  original  minus  selections.  The  return 
selections  are  accordingly  less  eflficient.  Nevertheless,  after  the  sixth 
return  selection  we  find  that  1  in  6  of  the  offspring  have  plus  grades  and 
their  average  is  lower  (that  is,  less  minus)  than  the  offspring  in  the  minus 
series  after  a  single  generation  of  selection.     (Cf.  Tables  16  and  36.) 


RETURN   SELECTION.  15 

The  amount  and  persistency  of  the  reversed  regression  in  this  series 
show  clearly  that  return  selection  is  not  easier  or  more  rapid  than  the 
original  modification  of  the  race  by  selection,  but  that  selection  in  either 
a  plus  or  minus  direction  has  cumulative  and  permanent  effects. 

Further  support  for  this  conclusion  is  furnished  by  return  selections 
(one  each)  made  from  the  seventh  generation,  from  the  eighth  genera- 
tion, and  from  the  eleventh  generation  of  the  minus  selection  series. 
(See  Tables  37, 38,  and  39.)  Generation  7  (Table  22)  was  produced  by 
parents  of  average  grade  —2.01.  Their  offspring  were  of  average 
grade  — 1.73,  a  regression  (toward  0)  amounting  to  0.28.  Certain  pairs 
of  these  offspring  of  grade  —0.75  and  —0.87  (mean  —0.78)  constitute 
the  return  selection  from  generation  7  (Table  37).  They  had  33  off- 
spring of  average  grade  —1.15,  a  regression  away  from  0  amounting  to 
0.37. 

Generation  8  of  the  minus-selection  series  (Table  23)  was  produced 
by  parents  of  mean  grade  —2.05.  Their  offspring  were  of  mean  grade 
— 1.80,  a  regression  (toward  0)  of  0.25.  Certain  pairs  of  these  offspring 
of  grades  —0.50,  —0.62,  and  —1.00  (mean  —0.72),  when  chosen  as 
parents,  produced  41  young  of  mean  grade  —1.51,  a  regression  away 
from  0  amounting  to  0.79.     (See  Table  38.) 

Generation  11  of  the  minus  series  (Table  2G)  was  produced  by  parents 
of  mean  grade  —2.30.  The  offspring  were  of  mean  grade  —2.15,  a 
regression  of  0.15  toward  0.  A  pair  of  the  offspring  of  mean  grade 
— 1.62  (Table  39)  produced  16  young  of  mean  grade  — 1.95,  a  regression 
of  0.32  away  from  0.  This  result  shows  that  the  selected  race  had  now 
passed  the  point  represented  by  the  grade  of  the  parents  (  —  1.62)  and 
the  offspring  regressed  toward  a  racial  mean  as  advanced  as  the  most 
extreme  individuals  obtained  previous  to  selection. 

To  show  that,  in  the  plus  selection  series,  a  return  selection  has  a 
result  similar  to  that  just  described,  two  experiments  may  be  cited: 

The  sixth  generation  of  the  plus  selection  series  was  produced  by 
parents  of  mean  grade  3.52,  and  their  offspring  were  of  mean  grade  3.11, 
a  regression  toward  0  amounting  to  0.41.  Certain  of  these  offspring 
of  mean  grade  2.00,  when  chosen  as  parents,  produced  17  young  of 
mean  grade  2.36,  a  regression  away  from  0  amounting  to  0.36.  (See 
Table  40.) 

The  eleventh  generation  of  the  plus  selection  series  (Table  11)  was 
produced  by  parents  of  mean  grade  —3.97;  their  offspring  were  of  mean 
grade  —3.78,  a  regression  of  0.19  toward  0.  Certain  of  these  offspring, 
ranging  in  grade  from  —2.62  to  —3.25  (Table  41),  mean  —2.79,  pro- 
duced 53  young  of  mean  grade  —3.32,  a  regression  away  from  0  amount- 
ing to  0.53.  The  regression  in  this  case,  as  in  all  those  previously 
described,  was  toward  the  racial  mean  of  the  previous  generation,  which, 
however,  it  has  in  no  case  reached. 


16  PIEBALD    RATS   AND    SELECTION. 

This  can  have  but  one  meaning.  The  genetic  character  of  the  hooded 
rat  is  in  a  general  way  correctly  indicated  by  its  somatic  character. 
Selection  is  therefore  immediately  effective,  whether  plus  or  minus  in  char- 
acter and  whether  or  not  preceded  by  selection  in  the  same  direction  or  in  an 
opposite  direction.  But  regression  may  be  expected  from  the  character 
of  aberrant  parents  back  toward  the  normal  of  the  previous  generation, 
yet  this  regression  will  in  general  be  less  than  the  departure  of  the 
aberrant  parents  from  the  normal  of  their  generation.  If  one  desires 
in  such  a  case  to  obtain  continuous  and  progressive  departure  from  the 
normal  in  either  a  plus  or  a  minus  direction,  he  need  only  select  con- 
tinuously in  the  desired  direction. 

CROSSES  WITH  WILD  RATS. 

As  a  further  test  of  the  permanency  of  the  modification  effected  by 
selection  in  the  hooded  pattern  of  rats,  crosses  have  from  time  to  time 
been  made  of  the  selected  races  with  a  pure  wild  stock,  i.  e.,  with  ordi- 
nary wild  animals  caught  in  traps.  In  all  cases  the  wild  animals  used 
were  known  to  be  homozygous  as  regards  gray  coat  and  self  pattern, 
since  when  crossed  with  black-hooded  animals  they  produced  only  gray 
self  offspring.  In  such  crosses  the  hooded  pattern  is  recessive,  the  Fi 
offspring  being  indistinguishable  from  ordinary  wild  gray  rats  except 
for  the  possession  of  a  white  patch  of  varying  size  upon  the  belly,  but 
even  this  may  be  lacking.     (See  Plate  2,  d^  8000,  8018,  and  8021.) 

The  grade  of  the  hooded  young  extracted  from  a  cross  with  wild  ani- 
mals corresponds  in  a  general  way  with  the  grade  of  the  hooded  animal 
used  in  making  the  cross,  as  the  following  cases  will  show.  (Compare 
also  Plates  2  and  3.) 

A  female  of  grade  —1.87,  belonging  to  generation  2\,  minus  series 
(compare  Tables  2  and  3),  was  crossed  with  a  wild  male.  (See  Plate  2, 
9  6176.)  Among  her  Fo  descendants  (cf .  Plate  2, 8070  to  8078)  occurred 
62  hooded  individuals,  whose  grade  distribution  is  shown  in  Table  42, 
first  row.  Their  mean  grade  is  +0.31,  although  the  uncrossed  race  of 
the  same  grade  and  generation  gave  offspring  of  mean  grade  about 
—  1.20.  The  cross,  therefore,  had  apparently  increased  the  pigmenta- 
tion of  the  extracted  hooded  recessives.  This  idea  is  supported  by  the 
result  of  a  control  mating  of  the  particular  female  used  in  making  the 
cross.  When  she  was  mated  with  a  hooded  male  of  the  same  grade  as 
herself,  she  produced  three  young,  all  of  grade  —1.00.  The  extracted 
recessive  grandchildren,  as  a  group,  show  greatly  increased  pigmenta- 
tion as  compared  with  this,  but  vary  greatl}^  in  the  extent  of  the  increase. 
Some  show  very  little  modification,  others  very  much,  the  most  extreme 
individual  being  of  grade  4-3.50.  It  was  undoubtedly  out  of  just  such 
modified  recessive  individuals  as  this  that  the  material  for  our  initial 
plus  selections  arose;  to  this  point  we  shall  return  later. 


CROSSES  WITH  WILD  RATS.  17 

The  F2  (or  second  generation)  offspring,  however,  include  about  1 
hooded  individual  in  4.  In  a  total  of  962  F2  young,  230  were  hooded, 
or  24  per  cent.  This  summary  includes  only  those  litters  in  which 
dominants  as  well  as  recessives  were  recorded.  In  many  litters  only 
the  hooded  young  were  recorded,  as  the  special  object  of  the  investiga- 
tion was  to  ascertain  whether  the  extracted  recessives  were  like  the  pure 
hooded  race  in  grade  or  not.  In  the  above  summary  also  the  hooded 
grandparent  was  in  every  case  a  female.  The  reciprocal  cross  is  more 
difficult  to  obtain,  but  one  wild  female  rat,  caught  in  1911,  has  bred 
quite  regularly  in  captivity,  though  each  time  she  has  murdered  her 
hooded  mate  prior  to  the  birth  of  the  young.  Her  Fo  grandchildren 
derived  from  matings  with  males  of  the  minus  series  include  32  hooded 
and  96  non-hooded  individuals,  exactly  25  per  cent  hooded. 

A  second  cross  of  selected  animals  of  the  minus  series  was  made 
between  a  wild  male  and  four  females  of  grade  —  2  derived  respectively 
from  generations  5j,  5^,  6^,  and  7.  As  a  group  these  mothers  are  more 
nearly  comparable  with  generation  6,  Table  21,  than  with  any  other 
uncrossed  group.  As  the  Fi  progeny  of  these  four  mothers  by  a  wild 
male  were  mated  i?iter  se,  it  is  possible  to  deal  with  their  hooded  grand- 
children only  as  a  group.  The  character  of  these  is  indicated  in  the 
second  row  of  Table  42.  They  number  48  individuals  and  have  a 
mean  grade  of  +0.25,  showing  a  modification  in  a  plus  direction  similar 
to  that  observed  in  the  previous  case. 

A  third  cross  in  which  the  minus  series  is  concerned  was  made  between 
females  of  grade  —2  and  —2.25,  generation  10,  and  wild  males.  The 
F2  offspring  include  91  hooded  individuals  classified  as  to  grade  in  the 
third  row  of  Table  42.  Their  mean  grade  is  +0.24,  confirming  fully 
the  results  obtained  in  the  two  previous  experiments. 

With  these  three  cases  we  may  compare  three  cases  in  which  animals 
of  the  plus  selection  series  were  crossed  with  a  wild  male.  (See  the  last 
three  rows  of  Table  42.)  Females  of  grade  +3.00,  generation  3,  were 
crossed  with  a  wild  male.  From  this  mating  resulted  21  hooded  grand- 
children, ranging  in  grade  from  +1.75  to  +3.50,  mean  +2.56.  These 
grandchildren,  it  will  be  observed,  in  no  case  are  of  minus  grade,  as  are 
about  half  the  grandchildren  when  the  grandparent  is  of  minus  grade. 
There  is  also  no  clear  evidence  of  modification  of  the  hooded  character 
by  the  cross  in  this  case.  The  grade  of  the  extracted  hooded  individual 
is  just  about  what  uncrossed  individuals  of  grade  +3.00  produced  in  the 
corresponding  generation  of  the  plus  series. 

In  the  next  case  two  females  of  the  plus  series,  belonging  to  genera- 
tions 5  and  6,  respectively,  were  crossed  with  a  wild  male  and  their 
children  were  bred  inter  se.  There  resulted  38  hooded  grandchildren, 
as  shown  in  the  next  to  the  last  row  of  Table  42.  The  range  of  the 
grades  of  these  hooded  grandchildren  was  similar  to  that  of  the  grand- 
children in  the  foregoing  case,  but  their  mean  was  somewhat  higher,  as 


18  PIEBALD    RATS   AND    SELECTION. 

we  should  expect,  since  they  are  descended  from  more  highly  selected 
individuals;  for  the  hooded  grandparents  in  this  case  were  of  grade 
+3.25  (generation  5)  and  +3.50  (generation  6),  whereas  the  grand- 
parent in  the  foregoing  case  was  of  grade  +3.00  (generation  3). 

It  is  a  noteworthy  fact  that  in  both  these  cases  the  wild  cross  does  not 
seem  to  have  increased  the  pigmentation  in  extracted  hooded  indi- 
viduals, as  it  did  when  the  minus  series  was  crossed,  but  rather  to  have 
diminished  it;  yet  the  difference  between  observed  and  expected  is  not 
great.  We  might  disregard  it  altogether,  if  a  similar  but  more  striking 
result  were  not  observable  in  the  third  case  as  well  as  in  another  series  of 
crosses  presently  to  be  described. 

The  third  case  (last  row  of  Table  42)  involves  a  cross  between  a 
female  of  grade  +4.25,  generation  10,  plus  series,  and  a  wild  male. 
The  F2  offspring  include  16  hooded  individuals  of  mean  grade  +3.15. 
Animals  of  this  grade  in  the  uncrossed  race  in  this  generation  produced 
young  of  mean  grade  +3.84. 

Before  leaving  this  subject  it  is  important  to  observe  the  considerable 
difference  between  the  extracted  hooded  grandchildren  of  the  minus- 
series  rats,  as  a  group,  and  those  of  the  plus  series.  The  latter  is  unmis- 
takably a  plus-series  group;  the  former  is  on  the  border  line  between 
the  two  series.     (Cf.  Plates  2  and  3.) 

CROSSES  WITH  BLACK  "IRISH"  RATS. 

As  a  control  on  the  results  given  by  the  wild  crosses,  we  may  examine 
the  results  obtained  by  crossing  the  plus  and  minus  selected  races  with  a 
black  Irish  race.  The  Irish  race  used  for  this  purpose  consisted  of  ani- 
mals black  everywhere  except  on  the  bell}^  On  the  system  of  grading 
used  in  this  paper  they  would  range  from  +4f  to  +5f ,  +6  being  an  all- 
black  rat. 

Crosses  of  minus-series  hooded  rats  with  Irish  produced  Irish  Fi  off- 
spring with  rather  more  white  on  the  belly  than  the  Irish  parents 
possessed.  In  the  F2  generation  hooded  individuals  reappeared  in 
approximately  the  expected  25  per  cent.  In  a  total  of  764  second  gen- 
eration young,  171,  or  22.4  per  cent,  were  hooded.  The  grade  of  pig- 
mentation of  these  extracted  recessives  as  compared  with  that  of  their 
hooded  grandparents  we  may  now  consider,  as  was  done  in  the  case  of 
the  wild  crosses.     (See  Table  43.) 

Six  individuals  of  the  minus  selection  series,  of  generation  3h,  and  of 
mean  grade  -1.50,  were  crossed  with  Irish  rats  producing  Irish  off- 
spring which  were  mated  inter  se.  Among  the  grandchildren  appeared 
the  usual  proportion  of  recessives  (hooded),  90  in  number.  The  dis- 
tribution of  these  as  regards  grade  of  pigmentation  is  shown  in  the  first 
row  of  Table  43.  Their  mean  grade  is  - 0.62,  that  of  uncrossed  hooded 
rats  of  the  same  grade  as  the  hooded  grandparents  being  - 1.31  in  gener- 


'i    T^,T.,T^    " 


CROSSES   WITH    BLACK    "  IRISH  '    RATS.  19 

ations  3  and  4.  In  other  words,  the  cross  has  considerably  increased  the 
pigmentation  in  the  hooded  grandchildren  over  what  was  to  be  expected 
had  the  cross  not  taken  place.  Nevertheless  the  increase  in  this  case  is 
less  than  in  the  similar  cross  with  wild  rats.     (Compare  Table  42.) 

In  the  second  row  of  Table  43  is  shown  the  grade  distribution  of 
extracted  hooded  grandchildren  of  two  mothers  of  grade  —1.87  and 
generation  4.  The  mean  of  the  53  hooded  grandchildren  is  in  this  case 
—  0.73,  that  of  uncrossed  hooded  parents  of  the  same  grade  and  genera- 
tion being  1.18.  This  average  is  probably  too  low.  An  examination 
of  the  means  of  adjacent  classes  (Table  19)  indicates  that  it  should  be 
about  1.35. 

In  the  third  row  of  Table  43  is  shown  the  grade  distribution  of  the 
extracted  recessive  grandchildren  of  a  —2.00  male,  minus-series  rat,  of 
generation  7|.  The  66  grandchildren  are  of  mean  grade  —0.94,  ex- 
pected —  1.75. 

Comparing  the  three  experiments  (first  three  rows  of  Table  43),  we 
see  that  the  more  advanced  grandparents,  in  grade  and  generations  of 
selection,  have  the  more  advanced  grandchildren;  but  in  every  case 
these  are  less  advanced  than  grandparents  of  the  same  sort  would  have 
given  had  they  not  been  crossed.  Hence  crossing  with  Irish  has  clearly 
had  the  effect  of  increasing  the  pigmentation  in  the  minus  series  in  the 
same  waj'-  (but  in  lesser  degree)  as  did  crossing  with  wild  animals. 

The  results  of  crossing  hooded  rats  of  the  plus  series  with  Irish  ones 
are  shown  in  the  last  two  rows  of  Table  43.  Several  rats  of  mean  grade 
+2.25  and  of  generation  2  were  crossed  with  Irish,  and  their  Irish 
young  were  then  bred  inter  se,  producing  239  hooded  grandchildren. 
These  ranged  in  grade  from  —1.00  to  +3.25,  their  mean  being  +1.27. 
The  grade  of  uncrossed  rats  of  like  grade  and  generation  to  the  hooded 
grandparents  is  +1.80.  Hence  here,  as  in  the  cross  with  wild  rats,  the 
pigmentation  has  not  been  increased,  but  decreased  by  the  cross,  con- 
trary to  what  we  should  expect.  Further,  the  departure  from  expecta- 
tion is  greater  in  this  cross  than  in  the  wild  cross.  These  conclusions 
are  supported  by  the  results  shown  in  the  last  row  of  Table  43.  In  the 
experiment  here  recorded  a +  3.00  rat  of  generation  3  was  mated  with 
an  Irish  rat.  The  hooded  grandchildren  derived  from  this  cross  were, 
as  shown  in  the  table,  of  mean  grade  +0.95,  expected  about  +2.50. 
Since  the  number  of  animals  recorded  in  this  experiment  is  compara- 
tively^ small,  the  quantitative  result  is  less  important  than  that  of  the 
foregoing  experiment,  but  qualitativeh^  the  two  are  in  entire  agreement. 

The  various  crosses  of  the  selected  minus  and  plus  series  v.'ith  wild 
rats  and  with  Irish  rats  respectively  are  consistent  with  each  other. 
In  every  case  the  cross  increases  the  pigmentation  of  the  minus  series  and 
decreases  that  of  the  plus  series;  in  other  words,  it  undoes  the  work  of 
selection  to  some  extent.  Does  this  mean  that  the  condition  created  by 
selection  was  in  reality  an  unstable  one,  so  that  an  outcross  tends  to  do 


20  PIEBALD    EATS   AND    SELECTION. 

away  with  it?  We  do  not  think  so,  but  to  this  question  we  shall  return 
again. 

The  question  might  be  asked  whether  the  modifications  produced  in 
the  selected  races  by  a  cross  with  wild  or  Irish  stock  are  likely  to  be 
more  or  less  permanent  than  those  produced  in  unselected  races  by  the 
same  means.  A  single  experiment  was  made  which  bears  on  this 
question  in  relation  to  the  Irish  cross.  One  of  the  —2.00  grandchildren 
recorded  in  the  third  row  of  Table  43  was  mated  with  —2.00  individuals 
of  the  uncrossed  stock  of  the  minus  series  and  produced  nine  young  of 
mean  grade  —0.63,  the  expectation  for  the  uncrossed  race  of  the  same 
grade  and  generation  being  about  —1.90.  In  other  words,  this  ex- 
tracted —  2.00  individual  regressed  (in  breeding)  as  if  it  really  had  been 
affected  by  the  cross,  even  though  it  did  not  show  it,  but  the  number  of 
young  is  so  small  that  no  emphasis  should  be  placed  upon  this  result. 

From  the  experiment  recorded  in  the  last  row  of  Table  43  were 
obtained  extracted  individuals  of  mean  grade  +1.37,  which  as  parents 
produced  16  young  of  mean  grade  +1.68,  or,  in  other  words,  offspring 
about  like  themselves.  Hence  the  changes  effected  by  a  cross  are  per- 
manent, like  those  effected  by  selection. 

PLUS  SELECTION  OF  "EXTRACTED  HOODED"  RATS. 

It  has  been  suggested  that  the  original  material  out  of  which  the 
plus  series  came  consisted  of  rnodified  individuals  produced  by  a  cross 
with  the  wild  race.  This  was  not  known  positively  to  be  so,  because 
part  of  the  original  stock  (with  which  MacCurdy  worked)  consisted  of 
hooded  black  and  hooded  gray  rats  captured  in  company  with  gray  self 
and  black  Irish  rats  and  albinos.  Subsequent  experiments  showed  that 
ordinary  albino  rats,  if  crossed  with  wild  gray  ones,  will  produce  in  F2 
all  these  classes  of  individuals.  This  indicated  pretty  clearly  that  the 
particular  colony  which  had  fallen  into  our  hands  had  probably  arisen 
by  the  crossing  of  an  escaped  albino  rat  with  wild  ones.  But  it  still 
remained  uncertain  what  sort  of  hooded  pattern  the  escaped  albino  had 
transmitted  and  w^hether  or  not  this  had  been  influenced  by  the  wild 
cross.  We  therefore  determined  to  ascertain  whether  out  of  our  minus 
series  crossed  with  wild  a  plus  series  could  be  derived.  To  this  end 
certain  of  the  F2  extracted  hooded  indi\dduals  (entered  as  grandchildren 
in  Table  42,  row  1,  and  descended  from  a  single  hooded  individual  of 
grade  —1.87,  generation  2^)  were  mated  inter  se,  thus  producing  an  F3 
generation.  Table  44,  second  row.  The  selected  individuals  were  the 
aberrant  male  of  grade  +3.50  and  females  of  grade  +1.50,  so  that  the 
mean  grade  of  the  chosen  parents  (extracted  from  the  crossed  minus 
series)  was  +2.50.  They  had  34  young  ranging  in  grade  from  0  to 
+3.50,  mean  +2.06,  a  regression  of  0.44  toward  0,  repeating  the  phe- 
nomenon regularly  found  in  both  selection  series. 

In  this  same  experiment  some  Fo  parents  of  mean  grade  -0.75  had 
19  young  (first  row  of  Table  44),  whose  mean  grade  was  -0.04,  a 


PLUS   SELECTION    OF    '^EXTRACTED   HOODED"    RATS. 


21 


regression  of  0.71  toward  0.  We  should  expect  the  regression  of  the 
offspring  of  such  parents  to  be  less  than  that  of  the  offspring  of  the 
—  2.50  parents,  and  so  it  would  be  if  it  were  not  for  one  aberrant  indi- 
vidual. Larger  numbers  of  offspring  would  undoubtedl}-  have  given 
the  expected  result. 

From  among  the  F3  offspring  were  chosen  parents  for  the  next  gen- 
eration (F4).  The  chosen  parents  ranged  in  mean  grade  from  +2.25  to 
+3.12  (Table  45),  average  +2.52.  They  produced  205  young  ranging 
in  grade  from  —0.25  to  +3.50,  mean  +1.86,  a  regression  of  0.66. 

The  parents  for  the  next  generation  (Table  46)  ranged  in  mean  grade 
from  +2.00  to  +3.00,  the  mean  being  +2.27.  They  produced  119  off- 
spring of  mean  grade  +2.06,  a  regression  of  only  0.21. 


Table  B. — Comparison  of  the  present  scries  with  the  more  general  plus  selection  series. 


Selection. 

Present  series. 

General  (plus)  series. 

Mean 
parents. 

Mean 
offspring. 

Regres- 
sion. 

No.  of 
offspring. 

Mean 
parents. 

Mean 
offspring. 

Regres- 
sion. 

No.  of 
offspring. 

1 

2.50 

2.06 

0.44 

34 

2.51 

2.05 

0.46 

150 

2 

2.52 

1.86 

.66 

205 

2.52 

1.92 

.60 

471 

3 

2  27 

2.06 

.21 

119 

2.73 

2.51 

.22 

341 

4 

2.69 

2.41 

.28 

194 

3.09 

2.73 

.36 

444 

5 

2.77 

2.32 

.45 

97 

3.33 

2.90 

.43 

610 

6 

3.08 

2.67 

.41 

45 

3.52 

3.11 

.41 

861 

The  parents  chosen  from  among  these  offspring  ranged  in  mean  grade 
from  +2.37  to  +3.25,  average  +2.69.  They  produced  194  offspring 
of  grade  +0.50  or  higher  (Fe,  Table  47),  the  range  for  the  first  time 
lying  wholly  in  the  plus  direction.  The  mean  grade  of  the  offspring  was 
+2.41,  a  regression  of  0.28. 

The  parents  of  the  next  generation  (F^,  Table  48)  range  in  mean  grade 
from  +2.62  to  +3.37,  their  average  being  +2.80.  Their  154  offspring 
range  from  +0.75  to  +3.75,  mean  +2.46,  a  regression  of  0.34. 

The  parents  of  the  last  generation  in  this  experiment  (Fg,  Table  49) 
were  of  mean  grade  +3.08.  They  produced  45  offspring  of  mean  grade 
+2.67,  a  regression  of  0.41. 

As  a  result  of  a  single  cross  with  a  wild  race  followed  by  six  successive 
selections,  a  narrow-striped  or  minus  family  has  thus  been  converted 
into  a  wide-striped  or  plus  family.  Considering  the  smaller  number  of 
offspring  from  which  selections  could  be  made,  progress  was  quite  as 
rapid  in  this  series  as  in  the  larger  plus  selection  series.  The  regression 
is  surprisingly  similar,  generation  by  generation,  in  the  two  series.  (See 
Table  B.)  But  it  seems  improbable  that  the  closeness  of  the  agreement 
has  any  significance.  This  series  has  the  theoretical  advantage  of  being 
derived  from  a  single  individual  of  the  minus  selection  series. 


22  PIEBALD    RATS   AND    SELECTION. 

CROSSES  OF  THE  PLUS  RACE  WITH  THE  MINUS  RACE. 

When  animals  of  the  plus  selection  series  are  crossed  with  animals  of 
the  minus  selection  series,  an  Fi  generation  of  offspring  is  obtained 
which  varies  about  a  mean  intermediate  between  those  of  the  respective 
uncrossed  races.  Thus,  from  an  examination  of  Table  50  it  will  be  seen 
that  when  —  2  animals  of  generation  6,  minus  series,  were  crossed  with 
+3.50  or  +3.75  animals  of  generation  5,  plus  series,  an  Fi  generation 
(Series  1)  was  obtained  consisting  of  93  animals  of  mean  grade  +0.0G. 
This  generation  is  rather  more  variable  than  either  uncrossed  race, 
its  standard  deviation  being  0.71.  The  same  is  true  of  a  second  set 
(Series  2)  of  crosses  made  between  a  male  of  grade  —3.25,  generation 
10,  minus  series,  and  females  of  grade  +3.75,  generation  10,  plus  series. 
The  14  Fi  offspring  are  of  mean  grade  + 1  and  have  a  standard  deviation 
of  0.60.     (See  Table  50,  Series  2.) 

In  both  the  series  of  crosses  summarized  in  Table  50,  the  Fo  genera- 
tion is  more  variable  than  Fi.  In  Series  1,  305  Fo  animals  are  recorded, 
having  a  standard  deviation  of  1.01  as  compared  with  0.71,  the  standard 
deviation  of  the  Fi  generation.  In  Series  2,  the  F^  offspring  number  73 
and  have  a  standard  deviation  of  0.87,  that  of  the  previous  generation 
being  0.60. 

The  mean  of  the  F2  generation  is  very  similar  to  that  of  the  Fi  genera- 
tion. In  Series  1,  the  mean  of  Fi  is  0.06,  and  that  of  F2  is  0.24;  in  Series 
2,  the  mean  of  Fi  is  1.00,  and  that  of  F2  is  0.72. 

It  may  also  be  seen  from  an  examination  of  Table  50  that  among  the 
Fi  offspring  produced  by  crossing  the  plus  and  the  minus  series  there  are 
differences  in  transmission,  as  there  are  in  the  expression  of  the  hooded 
pattern.  In  general  those  Fi  individuals  which  are  of  high  grade  pro- 
duce offspring  of  higher  grade  than  do  their  low-grade  brothers  and 
sisters.     This  is  exactly  what  has  been  observed  in  both  uncrossed  races. 

SUMMARY  OF  RESULTS. 

The  experiments  which  have  been  described  in  the  foregoing  pages 
have  shown  that : 

1.  The  hooded  pattern  of  rats  behaves  as  a  simple  Mendelian  char- 
acter in  crosses  with  either  the  Irish  pattern  or  the  wholly  pigmented 
condition  of  wild  rats. 

2.  Though  behaving  as  a  unit,  the  hooded  pattern  fluctuates— that 
is,  it  is  subject  to  plus  and  minus  variations. 

3.  Selection,  plus  or  minus,  changes  the  position  of  the  mean  and 
mode  about  which  variation  occurs. 

4.  The  results  of  such  plus  or  minus  selections  are  permanent,  for 
return  selection  is  not  more  effective  than  the  original  selection,  and 
during  return  selection  regression  occurs  away  from  the  original  mode, 
that  is,  toward  the  mode  established  by  selection. 


SUMMARY   OF   RESULTS.  23 

5.  During  the  progress  of  the  original  selection  (thirteen  successive 
generations)  variability  as  measured  by  the  standard  deviation  was 
somewhat  diminished. 

6.  Upon  crossing  the  selected  plus  and  minus  races  with  each  other, 
the  variability  was  somewhat  increased  in  Fi  and  was  further  increased 
in  Fa.  The  extreme  conditions  (plus  or  minus)  of  the  grandparents 
rarely,  if  ever,  recur  in  this  generation.  Only  one  individual  among  378 
F2  young  has  been  recorded  in  a  grade  as  extreme  as  either  grandparent. 

7.  Hooded  animals  extracted  in  F2  as  recessives  from  a  cross  with 
either  Irish  or  wild  rats  are  as  a  rule  more  variable  than  the  selected  race 
used  in  making  the  cross.  In  crosses  with  an  Irish  race  the  minus  series 
was  affected  in  like  measure.  In  crosses  with  wild  rats  the  variability 
of  the  plus  series  was  not  appreciably  affected  (in  two  experiments  it 
was  slightly  reduced,  and  in  one  experiment  it  was  slightly  increased). 
But  the  variability  of  the  minus  race  was  more  than  doubled  by  crosses 
with  wild  rats. 

8.  The  mean  of  the  minus  race  was  lowered  by  a  cross  with  either  the 
Irish  race  or  with  wild  rats,  but  more  extensively  by  the  latter.  The 
mean  of  the  plus  race  was  lowered  a  very  little  by  a  cross  with  wild  rats, 
but  considerably  by  a  cross  with  the  Irish  race. 

DISCUSSION. 

It  Y\^ould  be  possible  to  suppose,  as  the  senior  author  has  elsev/here 
suggested  (Castle,  1912),  that  the  Mendelian  unit  character  involved  in 
these  experiments  is  subject  to  quantitative  variation  and  that  such 
quantitative  variations  have  a  tendency  to  persist  from  generation  to 
generation.  This  would  account  for  the  effectiveness  and  permanency 
of  selection  when  brought  to  bear  upon  the  variations.  It  might  also 
form  a  basis  for  explaining  the  increased  variability  which  follows  cross- 
ing, this  being  regarded  as  due  to  contamination  in  the  heterozygote,  but 
there  are  certain  other  observed  effects  of  crossing  which  it  seems  impos- 
sible to  account  for  on  this  basis.  In  particular  it  is  observed  that  while 
crossing  the  minus  series  makes  it  less  minus  as  the  hypothesis  of  con- 
tamination would  demand,  crossing  the  plus  series  makes  it  less  plus,  the 
opposite  of  what  a  contamination  theory  would  demand.  For  we  can 
readily  understand,  on  the  basis  of  contamination,  how  a  +6  gamete 
being  combined  with  a  —2  gamete  might  change  the  latter  in  a  plus 
direction;  but  if  the  same  +6  gamete  is  associated  with  a  +4  gamete 
we  should  expect  it,  if  it  has  any  influence  at  all,  to  make  this  also  more 
plus,  but  the  observed  effect  is  the  opposite;  the  extracted  gametes  are 
less  plus  in  character. 

This  difficulty  is  met  by  an  alternative  explanation,  the  main  feature 
of  which  was  first  suggested  by  our  colleague,  Dr.  E.  M.  East,  viz,  that 
although  we  seem  to  be  deahng  with  a  single  unit  character  as  evidenced 
by  the  monohybrid  ratios  obtained,  nevertheless  the  modifications 


24  PIEBALD    RATS   AND    SELECTION. 

which  form  a  basis  for  selection  are  due  (in  part  at  least)  to  agencies 
transmitted  independent!}^  of  the  hooded  pattern  (not  forming  a  part  of 
the  same  unit  character),  and  which  may  be  present  in  Irish  as  well  as 
in  wild  rats.  By  crosses  with  such  rats  the  supposed  modifiers  may 
become  associated  with  the  hooded  pattern  in  extracted  recessive  indi- 
viduals and  so  increase  its  extent.  Such  increase  does  actually  occur  in 
experiment. 

The  hypothesis  of  modifiers  independent  in  transmission  of  the 
hooded  unit  will  account  for  the  fact  that  F2  is  more  variable  than  Fi 
when  crosses  are  made,  on  the  familiar  principle  of  recombination  of 
independent  factors.  It  w411  account  for  the  observed  effectiveness  of 
selection  on  the  ground  that  what  selection  accomplishes  in  the  plus 
series  is  the  isolation  of  homozygous  conditions  of  modifiers  at  first 
present  only  in  heterozygous  form,  and  that  what  it  accomplishes  in 
the  minus  series  is  the  isolation  of  conditions  homozygous  for  lack  of 
modifiers  (or  for  inhibitors)  of  pigmentation.  This  same  hypothesis 
will  account  also  for  the  observed  reduction  of  variability  during  the 
progress  of  selection,  for  as  soon  as  any  particular  modifier  attains  a 
homozygous  condition  in  the  race  it  will  cease  to  occasion  variability, 
and  as  more  and  more  factors  become  homozj-gous  the  variability 
should  accordingly  diminish  and  finally  disappear  altogether,  so  far  as 
it  is  due  to  internal  and  heritable  causes. 

At  this  point  the  hypothesis  of  modifiers  encounters  serious  diffi- 
culty, if  one  holds  the  prevalent  or  ''genotype"  conception  as  to  the 
nature  of  Mendelian  factors,  viz,  that  they  are  fixed  and  unchangeable 
and  not  subject  to  quantitative  variation,  but  only  to  combination  in 
different  ways  with  other  factors.  This  conception  has  been  presented 
very  clearly  by  Dr.  East  (1912).  Some  objections  to  this  view  had 
previously  been  stated  by  Castle  (1912)  and  need  not  here  be  repeated. 

If  we  assume  that  there  exists  at  the  outset  a  definite  number  of 
modifiers  and  that  these  possess  a  definite  and  unchanging  power  to 
modify,  then  it  is  evident  that  selection  can  do  nothing  but  secure  homo- 
zygous conditions  as  regards  the  presence  or  absence  of  these  modifiers. 
When  such  homozygous  conditions  are  secured,  selection  will  cease  to 
modify  the  race.  The  experiment  has  progressed  far  enough  to  show 
that  extensive  modification  through  selection  is  possible  without  any 
marked  falling  off  in  variability.  No  indication  is  observable  that 
selection  will  become  ineft"ective  before  an  all-black  rat  is  obtained  in 
the  plus  series  and  an  all-white  rat  in  the  minus  series.  A  nearly  all- 
black  race  of  rats  has  already  been  secured.  We  propose  to  continue 
the  experiments  until  demonstrative  evidence  is  obtained. 

If  the  fixed-factor  idea  as  regards  modifiers  of  the  hooded  pattern  is 
rejected,  there  remain  still  tw^o  possible  alternative  views  regarding 
them.  Either  we  may  consider  that  the  modifiers  vary  in  strength, 
that  is,  in  power  to  modify,  or  we  may  consider  that  new  modifiers  arise 


DISCUSSION.  25 

from  time  to  time,  which  selection  may  either  add  in  homozygous  form 
to  the  germinal  complex  or  reject  altogether  from  it.  If  we  assume 
that  the  modifiers  vary  in  strength,  we  shall  have  to  grant  also  the  pos- 
sibility that  the  character  modified,  the  hooded  pattern,  may  itself  vary 
in  strength  independently  of  its  modifiers.  For  evidence  see  the  de- 
scription of  the  "mutant"  series,  page  30.  This  assumption,  I  under- 
stand, would  be  unacceptable  to  those  who  hold  a  genotj'pe  conception 
of  heredity,  though  we  ourselves  can  offer  no  valid  objection  to  it. 

If,  on  the  other  hand,  we  admit  that  new  modifiers  or  inhibitors  are 
from  time  to  time  coming  into  existence  spontaneously,  and  that  selec- 
tion can  use  these  to  modify  the  pattern  either  in  a  plus  or  in  a  minus 
direction,  then  we  must  admit  that  selection  is  an  agency  of  real  creative 
power,  able  to  modify  unit  characters  indefinitely  so  long  as  physio- 
logical limitations  are  not  reached. 

Now  it  seems  to  us  probable  that  what  we  call  the  unit-character  for 
hooded  pattern  is  itself  variable ;  also  that ' '  modifiers  "  exist — that  is,  the 
extent  of  the  hooded  pattern  is  not  controlled  exclusively  by  a  single 
locahzed  portion  of  the  germ-cell;  otherwise  we  should  be  at  a  loss  for 
an  explanation  of  the  peculiar  results  from  crossing  plus  series  hooded 
rats  with  those  which  are  still  more  extensively  pigmented;  for  by 
such  crosses  the  pigmentation  is  rendered  not  more  extensive  but  less  so. 
This  result  we  can  explain  on  the  supposition  that  the  selected  plus 
series  has  accumulated  inoi^e  modifiers  of  the  hooded  pattern  than  the 
wild  race  contains,  so  that  a  cross  tends  to  reduce  the  number  of  modifiers 
in  the  extracted  hooded  individuals.  No  other  explanation  at  present 
offers  itself  for  this  wholly  unexpected  but  indubitable  result.  If  a 
different  one  can  be  found  we  are  quite  ready  to  discard  the  hypotheti- 
cal modifiers  as  a  needless  complication,  contenting  ourselves  with  the 
supposition  that  the  unit  character  for  hooded  pattern  is  itself  variable, 
and  that  for  this  reason  racial  change  in  either  plus  or  minus  directions 
may  be  secured  at  will  through  repeated  selection. 

We  have  been  led  to  adopt  tentatively  an  hypothesis  that  modifying 
factors  exist  independent  of  the  single  factor  for  hooded  pattern  (though 
both  the  factor  for  hooded  pattern  and  its  modifiers  may,  so  far  as  we 
can  see,  be  quantitatively  variable)  by  another  series  of  observations, 
which  will  now  be  described. 

THE  "MUTANT"  SERIES. 

In  the  tenth  generation  of  the  plus  selection  series  there  appeared  two 
individuals,  a  male  and  a  female,  of  considerably  higher  grade  than  any 
previously  recorded  in  this  series.  They  are  not  included  in  Table  10 
because  we  have  been  and  still  are  in  doubt  as  to  their  exact  nature  and 
think  it  best  to  give  a  separate  account  of  them.  If  entered  in  Table  10 
one  would  appear  as  a  5^  individual  born  of  3|  parents  (mean  grade), 
the  other  as  a  5f  individual  born  of  3f  parents  (mean  grade).     The 


26  PIEBALD    RATS   AND    SELECTION. 

nearest  individuals  in  grade  to  these  two  produced  by  the  same  group 
of  parents  are  of  grade  4^,  but  some  4^  parents  of  the  same  generation 
produced  two  offspring  of  grade  5.  (See  Table  10.)  Because  of  the 
marked  advance  in  grade  of  these  individuals  beyond  the  ordinary 
range  of  variation  in  the  series  we  called  them  ''mutants,"  without 
wishing  then  or  now  to  commit  ourselves  to  any  particular  theory  as  to 
their  nature  or  origin.  We  have  used  the  term  and  now  use  it  as  one  of 
convenience  merely.  The  two  ''mutant"  individuals  had  the  same 
father  and  their  mothers  were  sisters.  Their  pedigree  for  two  genera- 
tions is  as  follows: 

Mutants.  Parents.  Grandparents. 

9  295G, +3|1  |91939,  +3i 

9  2957, +3^1  lcfl817,  +3f 


c?47G3,  +5^ 
9  5153,  +51 


(91162, +3^ 
cf2963,  +4  Lisio,  +31 


The  mutant  male  was  mated  with  the  mutant  female  and  also  with 
other  females  of  the  plus  series,  with  the  results  shown  in  Table  51.  In 
every  case  the  young  fall  into  two  distinct  groups,  one  of  which  varies 
about  the  general  mean  of  the  plus  series  (approximately  3|),  while  the 
other  varies  about  the  father's  grade  as  a  mean  (approximately  5^). 

The  mutant  female  had  IG  young,  6  in  the  lower  group,  mean  3.87, 
and  10  in  the  upper  group,  mean  5.60.  (See  Table  51,  lowest  row.) 
The  other  females  had  in  all  114  young  almost  equally  divided  between 
the  two  groups,  58  in  the  lower  group,  mean  3.73,  and  56  in  the  upper 
group,  mean  5.45.  This  result  indicates  clearly  (what  the  sequel  also 
confirms)  that  the  male  mutant  transmitted  in  half  his  gametes  the  high 
grade  of  pigmentation  which  he  himself  manifested,  while  in  the  other 
half  of  his  gametes  he  transmitted  the  ordinarj-  condition  of  the  plus 
race  at  that  time.  In  other  words  his  "mutant"  character  behaved  as 
a  dominant  unit  in  relation  to  the  ordinary  condition  of  the  plus  race. 

It  is  evident  that  the  female  mutant  was  of  similar  constitution. 
This  being  the  case,  we  should  expect  three-fourths  of  the  offspring  of 
the  two  mutants  to  be  in  the  upper  group.  In  reality  10  of  their  16 
young  were  of  this  sort. 

The  male  mutant  was  mated  also  with  females  of  the  minus  series 
with  the  results  indicated  in  Table  52.  Again,  the  offspring  fall  into 
two  distinct  groups,  a  lower  and  an  upper.  The  lower  group  should  be 
comparable  with  the  result  obtained  in  Fi  when  the  plus  and  minus 
races  are  crossed  with  each  other.  (Compare  Table  50.)  Such  it 
proves  to  be.  It  includes  35  individuals  of  mean  grade  —0.49  and 
standard  deviation  0.77.  Series  2  of  Table  50  is  nearly  contempo- 
raneous with  this  experiment.  The  Fi  offspring  in  that  series  were  of 
mean  grade  - 1  and  standard  deviation  0.60. 


THE  '' mutant"  series.  27 

The  upper  group  of  offspring  (Table  52)  result,  we  may  suppose, 
from  a  mutant  gamete  (grade  about  5|)  united  with  a  narrow  series 
gamete  (grade  about  —2).  This  group  includes  31  individuals  varying 
closely  about  grade  4|,  and  with  a  standard  deviation  of  only  0.31. 
The  lower  average  grade  of  this  group  (4.43)  compared  with  the  similar 
group  of  Table  51,  which  had  a  mean  of  5.47,  shows  the  influence  of 
the  minus-series  gamete  upon  the  heterozygote  in  lowering  its  grade  by 
about  1.  Whether  the  plus-series  gametes  have  any  effect  upon  the 
grade  of  the  heterozygotes  recorded  in  the  upper  group  of  Table  51  is 
not  certain,  because  a  homozygous  group  of  mutants  has  not  yet  been 
established.  It  may  be  observed,  however,  that  one  individual  in  the 
upper  group  of  Table  51  was  of  grade  6  (colored  all  over),  and  it  is  pos- 
sible that  homozygous  "mutants,"  when  obtained,  will  approximate 
that  grade,  as  most  wild  rats  do.  Further,  a  comparison  of  Tables  51 
and  53  shows  that  mutant  heterozygotes  formed  by  crosses  with  the 
plus  series  are  of  slightly  lower  mean  grade  than  the  offspring  of  the  two 
mutants,  among  which  should  occur  both  homozygous  and  heterozygous 
mutants.  It  seems  probable,  therefore,  that  homozygous  mutants  will 
be  found  to  be  of  somewhat  higher  grade  than  heterozygous  ones. 

The  question  early  suggested  itself  to  our  minds,  will  these  "mutants" 
prove  to  be  mutants  in  the  sense  of  De  Vries?  Will  they  prove  to  be 
more  stable  than  the  modifications  ordinarily  secured  b}^  selection  in  our 
experiments?  To  test  this  matter,  we  have  raised  two  additional  gen- 
erations of  offspring  from  the  two  mutants  and  have  bred  a  second 
generation  of  offspring  from  each  of  the  four  groups  of  Fi  offspring 
recorded  in  Tables  51  and  52,  derived  from  matings  with  the  plus  and 
minus  races  respectively. 

The  F2  descendants  of  the  two  original  mutants  proved  very  similar 
to  the  Fi  descendants.  (See  Table  53.)  They  fall  as  before  into  two 
groups,  an  upper  and  a  lower.  The  former  includes  30  individuals  of 
mean  grade  5.52,  the  latter  2  of  mean  grade  3.37.  As  the  parents  of 
this  generation  were  taken  wholly  from  the  upper  group  of  offspring  of 
generation  Fi,  and  as  theoretically  that  group  should  contain  2  hetero- 
zygous individuals  to  one  which  is  homozygous  for  the  "mutant"  char- 
acter, it  is  to  be  expected  that  in  F2  more  than  three-fourths  of  the 
offspring  will  fall  in  the  upper  group.  For  any  pair,  one  member  of 
which  is  homozygous  for  the  mutant  character,  should  produce  only 
offspring  falling  in  the  upper  group;  and  offspring  falling  in  the  loiver 
group  should  be  produced  only  by  pairs  both  members  of  which  are 
heterozygous. 

The  upper  group  in  Fo  should  contain  a  larger  proportion  of  homozy- 
gous mutants  than  in  Fi,  and  since  the  parents  of  F3  were  chosen  from 
this  upper  group  of  F2  offspring,  it  is  not  surprising  that  the  11  F3  off- 
spring recorded  up  to  this  time  all  fall  in  the  upper  group.  The  mean 
of  this  upper  group  is  remarkably  constant  through  the  three  genera- 


28  PIEBALD    RATS   AND    SELECTION. 

tions,  and  the  variability  of  the  group  as  measured  by  its  standard 
deviation  is  also  low,  namely,  0.19.  This  indicates  that  the  mutant 
character  is  a  strongly  dominant  unit  in  relation  to  the  ordinary  condi- 
tion of  the  plus  series. 

Table  54  shows  the  character  of  the  F2  offspring  of  the  original  male 
mutant  mated  with  females  of  the  plus  series.  The  lower  group 
parents,  those  into  which  the  mutant  character  did  not  presumably 
enter  at  all,  produced  59  offspring  recorded  in  the  first  part  of  Table  54. 
Their  mean  grade  is  3.78  and  their  standard  deviation  0.33.  These  are 
very  close  to  the  constants  of  the  general  plus  series,  which  for  genera- 
tion 10  were  3.73  and  0.36,  respectively. 

The  second  division  of  Table  54  shows  the  character  of  the  young 
produced  by  the  Fi  parents  of  the  upper  group  (Table  51).  Such 
parents  are  supposed  to  have  received  a  ''mutant"  gamete  from  their 
father,  grade  about  5.50,  and  a  plus -series  gamete  from  their  mother, 
grade  about  3.75.  If  they  produce  gametes  of  these  same  two  sorts,  their 
offspring  should  also  fall  into  two  corresponding  groups;  in  fact  they  do. 
There  are  11  offspring  of  mean  grade  3.86  and  79  offspring  of  mean 
grade  5.50.  As  in  the  previous  generation,  the  two  groups  do  not 
approach  each  other  in  grade.  The  mean  and  standard  deviation  of 
the  lower  group  of  offspring  are  similar  to  those  of  the  plus  race.  The 
mean  of  the  upper  group  is  about  the  same  as  that  of  their  parents 
(upper  group  of  offspring.  Table  51),  namely,  5.50,  as  compared  with 
5.45;  their  standard  deviation  is  somewhat  lower,  namely,  0.15,  as  com- 
pared with  0.23.  This  result  indicates  that  the  "mutant"  character 
and  the  hooded  character  of  the  plus  series  segregate  from  each  other  in 
a  simple  way  without  modifying  each  other  appreciablj^  It  seems 
possible  that  they  contain  the  same  modifiers  (if  modifiers  are  present) 
and  differ  merely  by  the  main  unit  which  we  called  the  hooded  character 
in  the  early  part  of  this  paper.  Each  contains  a  different  condition  of 
that  main  unit.  Consequently  there  is  no  increase  of  variability  in  F2 
when  these  two  conditions  are  intercrossed.  This  we  should  expect  to 
happen,  if  they  differed  by  more  than  a  single  factor. 

A  very  different  result  is  obtained  from  the  cross  between  the  mutant 
and  narrow  races.  Although  Fi  from  that  cross  was  quite  variable 
(see  Table  52),  F2  is  still  more  variable  (see  Table  55).  The  lower 
group  Fi  individuals,  which  resembled  Fi  between  the  plus  and  minus 
races,  produced  61  young  (first  division  of  Table  55),  which  resemble 
F2  between  the  plus  and  minus  races.  They  range  in  grade  from  —2 
to  +31,  mean  +0.58,  standard  deviation  1.17.  In  the  two  series  of 
crosses  between  the  plus  and  minus  races  (Table  50)  the  means  were 
+0.24  and  +0.72,  respectively,  and  the  standard  deviations  1.01  and 
0.87.  This  indicates,  as  did  the  cross  with  the  plus  series,  that  the 
"lower  group"  gametes  produced  by  the  original  mutant  male  did  not 
differ  materially  from  gametes  produced  by  the  ordinary  plus  race  from 
which  the  mutant  sprang. 


THE  "mutant"  series.  29 

The  second  division  of  Table  55  shows  the  character  of  the  F2  young 
produced  by  the  upper  group  of  Fi  offspring  recorded  in  Table  52.  It 
consists  of  two  groups,  a  lower  and  an  upper.  The  lower  represents 
the  extracted  minus  race,  the  upper  represents  the  extracted  dominants 
or  mutants,  whether  homozygous  or  heterozygous.  The  former  group 
lias  an  average  of  +0.75  and  a  standard  deviation  of  1.03,  which  values 
are  close  to  the  corresponding  constants  of  Series  2,  Table  50,  the 
latest  of  the  plus-minus  crosses,  in  which  the  mean  was  +0.72  and  the 
standard  deviation  0.87. 

The  upper  group  offspring  of  Table  55,  second  division,  the  homo- 
zygous and  heterozygous  mutants,  number  68;  they  have  a  mean  grade 
of  4.77  as  compared  with  4.43  in  Fi,  which  consisted  exclusively  of 
heterozygotes.  This  shows  the  extracted  homozygotes  to  be  of  higher 
grade  than  the  heterozygotes.  The  highest  grade  mutant  among  the 
31  Fi  young,  all  of  which  were  heterozygotes,  was  of  grade  5,  but  among 
the  68  Fo  young  are  16  of  higher  grade  than  5.  We  expect  one-third  of 
these  68  individuals  to  be  homozygotes.  Now  all  of  the  F2  mutants 
from  the  cross  of  mutant  with  plus  race  (Table  54)  were  of  grade  5  or 
higher,  only  2  in  79  being  as  low  as  5,  and  13  of  the  79  being  of  grade  5f , 
a  grade  not  attained  at  all  in  Fo  from  the  mutant-minus  cross  (Table  55) . 
This  result  shows  us  that  the  cross  with  the  minus  race  does  affect  per- 
manently the  mutant  character,  lowering  its  grade  even  in  homozj^gous 
mutants  extracted  from  the  cross.  It  also  increases  the  variability  of 
the  mutants,  for  the  standard  deviation  of  the  mutant  group  in  Table 
55  is  0.44,  whereas  in  Table  54  (mutant-plus  F2),  in  a  like  number  of 
individuals,  it  was  0.15,  or  only  about  one-third  as  great. 

That  the  variability  of  the  mutants  is  unaffected  by  a  cross  with  the 
plus  race,  but  that  it  is  increased  by  a  cross  with  the  minus  race,  and 
that,  further,  the  mean  of  the  mutants  is  affected  little  or  none  by  a  cross 
with  the  plus  race,  but  that  it  is  lowered  by  a  cross  with  the  minus  race — 
these  several  facts  are  all  conformable  with  the  hypothesis  that  the 
change  in  variability  due  either  to  crossiiig  or  to  selection  results  from  77iodi- 
fyiiigfactorswliich,  as  they  are  independent  of  the  main  factor  concerned, 
are  probably  transmitted  in  a  different  part  or  component  of  the  germ- 
cell  than  that  factor.  For  if  the  mutant  and  the  plus  race  are  alike  as 
regards  the  modifiers,  but  differ  only  in  the  main  factor,  then  no  change 
in  variability  should  result  from  intercrossing  them,  but  only  alterna- 
tive conditions  as  regards  the  main  factor.  This  is  the  observed  result. 
But  if  the  mutant  and  the  minus  race  differ  not  onh^  in  the  main  factor, 
but  also  in  modifiers  which  are  independent  of  it,  then,  when  they  are 
crossed,  we  may  expect  that  through  independent  segregation  of  main 
factor  and  modifiers  the  extracted  minus  race  will  be  raised  in  grade, 
while  the  extracted  mutants  are  lowered,  and  both  will  become  more 
variable.     This  also  is  the  observed  result. 

One  objection  may  be  offered  to  this  interpretation,  namely,  that 
the  increased  variability  is  not  delayed  until  Fs,  but  is  already  in  evi- 


30 


PIEBALD    RATS   AND    SELECTION. 


dence  to  some  extent  in  Fi.  The  same  thing  was  observable  in  the 
crosses  of  the  plus  and  minus  series  (Table  50).  From  that  table, 
Series  1,  it  will  be  observed  that  when  the  plus  and  minus  races  had 
standard  deviations  of  0.49  and  0.50,  respectively,  their  Fi  offspring  had 
a  standard  deviation  of  0.71,  an  increase  by  nearly  one-half;  F2  showed 
a  further  increase  to  1.01.  In  series  2,  Table  50,  the  uncrossed  races 
(generation  10)  had  standard  deviations  of  0.36  and  0.24;  their  Fi  off- 
spring had  a  standard  deviation  of  practically  twice  this,  namely  0.60; 
F2  showed  a  further  increase  to  0.87. 

At  the  time  of  the  mutant-minus  race  crosses,  the  minus  race  (genera- 
tion 10)  had  a  standard  deviation  of  0.24,  the  plus  race  of  0.36.  Fi 
(lower  group)  had  a  standard  deviation  of  0.77,  and  F2  of  1.17.  F^ 
mutants  (upper  group)  had  a  standard  deviation  of  0.31  which  rose  in 
F2  to  0.44.  These  various  facts  will  perhaps  be  better  grasped  if  pre- 
sented in  tabular  form: 

Table  C. 


Standard  deviation  of 

races  crossed,  same 

generation. 

S.  D. 
Fi 

S.  D. 
F2 

Plus-minus  cross,  series  1  (Table  50) 

Plus-minus  cross,  series  2  (Table  50) 

JIutant-minus  cross,  lower  group 

Mutant-minus  cross,  upper  group 

Mutant-plus  cross,  lower  group 

0.49 
.36 
.25 
.19 
.25 
.19 

0.50 
.24 
.24 
.24 
.36 
.36 

0  71 
.60 
.77 
.31 
.24 
.23 

1.01 
.87 

1.17 
.44 
.35 
.15 

Mutant-plus  cross,  upper  group 

The  mutant-plus  cross,  it  will  be  observed,  shows  no  increase  of  vari- 
ability either  in  Fi  or  in  F2,  but  crosses  involving  the  minus  race  show 
increase  of  variability  both  in  Fi  and  in  Fo.  Interpreted  on  a  jVIendelian 
basis,  this  means  that  the  mutant  and  plus  races  on  the  one  hand  and  the 
minus  race  on  the  other  hand  differ  by  more  than  a  single  factor.  If 
they  differed  by  only  a  single  factor,  then  crosses  between  them  should 
bring  no  increase  of  variability,  either  in  Fi  or  in  F2.  This  appears  to 
be  true  as  regards  the  mutant  and  plus  races  when  crossed  with  each 
other.  But  if  the  races  crossed  differ  by  more  than  one  factor,  and  if, 
further,  neither  parent  is  homozygous  as  regards  the  factors  in  which 
they  differ,  then  we  may  expect  an  increase  in  variability  both  in  Fi  and 
in  F2.  This  is  exactly  what  we  observe  when  the  minus  race  is  crossed 
with  either  the  plus  race  or  its  derivative,  the  mutant  race. 

If  we  suppose  that  the  plus  race  and  the  minus  race  differ  from  each 
other  by  certain  "modifiers,"  we  can  not  suppose  that  the  plus  and  the 
mutant  races  differ  by  these  same  modifiers.  They  differ  in  some  other 
single  respect;  perhaps  that  in  which  they  differ  is  the  mai7i  hooded 
factor.  Ai-e  we,  then,  to  suppose  that  the  plus  and  the  minus  races  do 
not  differ  as  regards  this  same  main  factor?     This  can  not  be  stated,  but 


THE    ''mutant"    series.  31 

we  see  no  reason  for  considering  them  identical  as  regards  that  factor. 
It  appears  that  the  mutant  race  arose  from  the  plus  race  by  a  single 
large  plus  variation,  which  seems  to  have  its  determiner  in  some  single 
component  of  the  germ-cell.  But  the  fact  that  this  change  came  as  a 
large  quantitative  variation  does  not  show  that  small  variations  are 
impossible  in  that  same  cell  component.  It  seems  to  us  quite  improb- 
able that  the  plus  mutation  could  have  arisen  in  the  minus  selection 
series.  We  believe  that  the  repeated  selection  which  was  practised  had 
something  to  do  with  inducing  this  change  in  the  plus  direction.  If 
one  can  increase  at  will  the  "modifiers"  which  make  the  pigmentation 
more  extensive,  it  does  not  seem  strange  that  after  a  time  a  readjust- 
ment should  occur  within  the  cell  which  should  incorporate  modifiers 
in  that  part  of  the  cell  which  is  responsible  for  the  unit-character 
behavior  of  the  hooded  pattern.  This  would  amount  to  a  quantitative 
change  in  the  unit-character  for  hooded  pigmentation. 


BIBLIOGRAPHY. 

Castle,  W.  E. 

1905.  Heredity  of   coat    characters  in  guinea-pigs  and  rabbits.     Carn.  In.st.  Wash. 

Pub.  23. 
1900.  The  origin  of  a  polydactylous  race  of  guinea-pigs.     Carn.  Inst.  Wash.  Pub.  49. 
1912.  The  inconstanc}'^  of  unit-characters.     American  NaturaUst,  vol.  46,  pp.  352-362. 
Castle,  W.  E.,  and  Alexander  Forbes. 

1906.  Heredity  of  hair-length  in  guinea-pigs  and  its  bearing  on  the  theory  of  pure 

gametes.     Cam.  Inst.  Wash.  Pub.  49. 
DeVries,  H. 

1901-1903.  Die  Mutationstheorie.     Veit  &  Co.,  Leipzig. 
East,  E.  M. 

1912.  The  mendelian  notation  as  a   description   of   physiological   facts.     American 

Naturalist,  vol.  46,  pp.  633-655. 
Jennings,  H.  S. 

1909.  Heredity  and  variation  in  the  simplest  organisms.     American  Naturalist,  vol. 

43,  pp.  321-337. 

1910.  Experimental  evidence  on  the  effectiveness  of  selection.     American  Naturalist, 

vol.  44,  pp.  136-145. 
Johannsen,  W. 

1909.  Elemente  der  exakten  Erblichkeitslehre.     G.  Fischer,  .Jena. 
MacCurdy,  H.,  and  W.  E.  Castle. 

1907.  Selection  and  cross-breeding  in  relation  to  the  inheritance  of  coat-pigments  and 

coat-patterns  in  rats  and  guinea-pigs.     Carn.  Inst.  Wash.  Pub.  70. 
Pearl,  R. 

1913.  Genetics  and  breeding.     Science,  n.  s.,  vol.  37,  pp.  539-546. 


32 


PIEBALD    RATS   AND    SELECTION. 


TABLES 


Table  l.—Classifwalion  of  the  first  generation  of  offspring  in  the  plus  selection  series.  At 
the  head  of  each  column  is  indicated  the  grade  of  the  individuals  recorded  in  that  coiuinn. 
The  figures  in  the  body  of  the  table  indicate  the  manbcrs  of  offspring  of  the  several  grades 
indicated. 


Grade  of  parents. 

Gr: 

<le 

of  offspring. 

Totals. 

Means. 

Regres- 
sion. 

+  1 

u 

H 

If 

2 

2\ 

2i 

2| 

3 

n 

1 
4 

1 

3 

1 

1 

1 
4 

1 

4 

1 

7 
18 

1.82 
1.76 



.05 
.24 

2       

2i 

21 

3 

1 

3 

1 

6 

2 

2 

20 

1.87 

.38 

2J                   

2i 

5 

3 

1 

13 

8 

1 

12 

5 
2 

1 

1 

9 

37 

5 

51 

2.06 
2.15 
2.12 

.44 
.47 
.63 

21 

2f 

7 

....!      3 

2 

17 

25 

3 

1 

2 

3 

3   

3 

12 

2.35 

.65 

Totals  or  means,  2.51 

i'"1 

20 

3 

13 

8 

44 

7 

31 

10 

14 

150 

2.05 

.46 

Table  2. — Classification  of  offspring  in  generation  2,  plus  selection  series. 


Grade  of  parents. 

Grade  of  ofif.spring. 

r/1 

■A 

a 
a 

1 

c 
_o 

"co 

T. 

;-> 

-1 

-1 

1 

—  2 

-io+i 

h 

3 

4 

1    li 

Ul|2 

[ 
2i2i2|  3 

3^3^ 

1    '^ 

31;  1 

2 

1 

1 

....  1 

....    2 

: 

2     5 
12  :  8 

7  ;  3 
5  1  5 

7 
8 

1 
7 

91 

8 
2 
1 
8 

2 

1 

7 

3      2 

1 

...     40 

...'    56 

1.70 
1.28 
1.87 
1.92 
1.80 
2.11 

1  92 

2  41 
2.47 
2.50 

.30 
.83 
.38 
.45 
.70 
.51 
.83 
.46 
.53 
.62 

.60 

21 

2\ 

1 

1 

3 

2 
1 

1 

1 

4 
19 

6 

3 

3 

9 

11 

g 

2f 

1 

22 

3 

1 
16 

1 

1 

45 

21 

4 
3 

13  i  7  in  i  0 

1 
1 

...    133 

...1    44 

52 

21 

1 

2 

6     5 
5     9 

1      3 

7 

Q  i  fi 

3 

1 
1 

11 

2| i 

1 

4     2 

6  9     6 
5     3     9 

7  4  '  ■; 

2| 

1 

i     93 

3. 

1 

1 
1 

1 

2 

14 

2 

6 

3 

1       59 
5 

31 

1 

1 

1 

...    1    ...    2 

i             '             i 

Totals  or  means,  2.52 

1 

1 

2 

!      t 
14       1 

5 

16 

45  37 

45  45  77 

1             1 

57  44  148  30 

1      1      1 

9 

3 

1     471 

1.92 

TABLES. 


33 


Table  3. — Classification  of  offspring  in  generation  3,  plus  selection  series. 


Grade  of  parents. 

Grade  of  offspring. 

03 

•— » 
c3 

O 

03 

i 

d 
o 

OJ 

bO 

f 

1 

u 

u 

If 

2 

2i 

2i 

2f 

3 

3i 

3f 

3f 

4 

21   

1 

1 

1 
5      6 

4 

1 

3 

21 

2.06  ; 

.06 

21 

21           

1 
5  j    5 

5  j    5 

3      9 

1      7 

1 

10 
4 

4 
10 

2 
9 

1 

17    i    9.    l.n 

.22 
.18 
-.02 
.29 
.17 
.51 
.05 
.08 
.46 

2\  

1 

2      1 

2 

1 
2 

1 

7  1    2 

54 
64 

2.32  j 

2.64  ' 

2.46 

2.70 

2.49 

3.07 

3.17 

2.91 

2f                      

2 

15  j  18 
7       7 

3 
1 
9 
3 

1 

1 
1 

1 

2 

21  

10      7 

43 

47 

21 

1 

7 
1? 

3 

14 

7 
U 

9 

17 

2 

1 
1 

10 

11 

5 

2 

3  

2 

1 

71 

3J 

! 

2 
1 

10 
6 

31                   

2 

31     

1 

2 

3       2 

8 

1 

Totals  or  means,  2.73 .  . 

1 

2 

5 

8 

20  j  51 

46     47 

66 

63 

21 

6 

4 

1     341 

2.51 

.22 

Table  4. — Classification  of  offspring,  generation  4,  plus  selection  series. 


Grade  of  parents. 

Grade  of  offspring. 

Totals. 

Means. 

d 
.2 

o 

f  i  1 

'-' 

11 

If 

2    2i 

21 

2f 

3 

3i 

3§ 

3f 

2| 

2 

2 

.... 

4 

3 



11 

2.55 

.05 

2 

21 

2f 

3 

3 
3 
25 
14 
6 
6 
1 

2 
3 
19 
21 
8 
2 

3 
7 

35 

28 

13 

3 

2 

4 

15 
29 
30 
17 
9 
3 

3 
4 
9 
11 
9 
2 
6 

18 
37 
143 
122 
64 
23 
20 

2.65 
2.97 
2.60 
2.69 
2.89 
2.70 
3.02 

.10 
.10 
.40 
.43 
.36 
.67 
.48 

* 

21      .                            ..    ..i---- 

1 

3 
4 

2 
8 

2 
1 

2 

1 

3 

1 

1 
1 

7     12 

2     11 

....     1 
1 
1       .. 

31 

31 

! 

31 

1 
1 

3* 

3 

2 

3 

31     . .    . 

31 

3 

1 

1 

1 

6 

2,75 

1.00 

1 

j 

Totals  or  means,  3.09 

1      1 

1       2 

12 

30 

60 

58 

96 

110 

45 

19 

9 

444 

2  73 

36 

34 


PIEBALD    RATS   AND    SELECTION. 


Table  5.— Classification  of  generation  5,  plus  selection  series. 


Grade  of  parents. 

Grade  of  offspring. 

Totals. 

03 

a 

d 
0 

'm 
m 

V 

bO 

<u 

3 

1 

li 

u 

n 

2 

2i 

^2 

2f 

3 

3i 

3A 

31 

4 

41 

93 

1 

2 

4 

3 

20 

24 

25 

14 

7 

2 

4 

2 

2 

29 

39 

36 

16 

16 

4 

1 

1 

1 

4 

3 

3 

18 

27 

28 

8 

12 

3 

1 

1 

2 
1 
8 
7 
15 

11 
2 

1 
1 

12 
21 

15 
114 

3.00 

2.87 
2.81 
2.81 
2.81 
2.94 
2.86 
3.08 
3.07 
3.35 
3.36 
3.00 

-.25 
0 
.19 
.31 
.44 
.43 
.64 
.54 
.68 
.52 
.64 
1.12 

^* 

9' 

4 
1 
9 
6 
14 
5 
3 

4 

5 

14 

14 

11 

10 

5 

2 

1 

2 

•^8 

3-i-              i... 

1 
1 

1 

1 

1 
1 

1 
4 

1 

8 
12 
5 
5 
3 

3 

"7 
3 
4 

1 
2 
2 

1 

1 
2 

3i  

1 

138 

145 

69 

3f     

iih 

"2 

3|               

3 

64 

14 

8 

34 

31            

1 

1 

2 
1 
1 

4 

...      7 

...;    3 

4i 

1 

Totals  or  means,  3.33 

1 

2 

2 

2 

6 

35 

44 

66 

101 

151 

107 

57 

24 

U 

1 
1     610 

2.90 

.43 

Tabi^  6. — Classification  of  generation  6,  plus  selection  series. 


Grade  of  parents. 

Grade  of  offspring. 

Means. 

Regression. 

1^ 

If 

2 

2i 

21 

2f     3 

3i 

31    3f 

4 

4i 

41 

0 

1      i 

1  

1 

2 
28 
143 
123 
212 
181 

31 

3 

1 

4 
3 
5 
9 
4 
2 
4 
1 

1 

3 
6 
4 
8 

1 

1 

8 

13 

15 

11 

3 

3 

4        7 
26      31 

5 

3 

24 

2.84 
3.10 
2.96 
3.10 
3  16 
3.22 
3.26 
3.41 

.28 
.15 
.41 
.40 
.46 
.53 
.61 
.59 

3i      

1 
1 

1.<> 

1 

3| 

20      28      30 
29  i    49      41 

24       34  1    36 

14  \    5 
43  j  17 
37     21 
20     13 
20  j  14 
6      4 

3i 

4 
5 
2 
4 

1 

3f 

1 

31 

1 

1 

4  18 

5  14 
...  i      2 

13 
15 

1 

78 
80 
14 

3| 

4 

.  .  .  . 

Totals  or  means,  3.52. . . 

3 

6 

32 

24 

54 

113     183 

172 

166 

90 

17,....     1  '  861 

3.11 

.41 

TABLES. 

Table  7. — Classification  of  generation  7,  plus  selection  series. 


35 


Grade  of  parents. 

Grade  of  offspring. 

m 

CO 

§ 

2 

H 

If 

2 

21 

2i 

2f 

3  I3J 

3h 

3| 

4 

1 
4i  41  4| 

1         1 

31 1. ..!.... 

2      2 
6  1  11 

4      5 

7  i  11 

7      4 
30  i  19 

3 

22 
21 

35 
75 
58 
23 

2 

1 

1 

23 
20 
34 
49 
42 
21 
4 
1 

1 

28     '  "7 

.25 
.16 

.40 

.32 

.39 

.40  1 

.38; 

.47  1 

.50 

3^ l!.... 

31 1      2 

3J 

1 
4 
3 

7 
4 
6 
3 
1 

; 

131 

160 

177 

289 

184 

90 

15 

3 

3.09 
2.97 
3.18 
3.23 
3.35 
3.49 
3.53 
3.75 

10 
6 

18 
9 

11 
3 
1 

17 
9 

13 
3 
2 
1 

21 
21 
26 
16 
4 
1 

27 
31 
47 
28 

18 
28 
53 
9'i 

1 

1 
1 
3 
6 

1 

1 

3f 

7 

.  .  .  .|.  .  .  . 

i 

3^ 

2 
2 

i 

3| ' 

10     12 

3  1.... 

2 

1 
1 

4 ! 

41 ! ! 

i         \ 

... 

Totals  or  means,  3.56. . 

1             1             !       1             1 
2  1    2     35  1  55     56   103  il83   159  i240 

195 

29 

12 

2  1   2 

1077     3.20 

.36  ' 

Table  8. — Classification  of  generation  8,  plus  series. 


Grade  of  parents. 

Grade  of  offspring. 

CQ 

J 

1 
m 

K 

Qi 

0 

If 

2 

01    01    03 

i       ! 

3     3J    3^ 

3f 

4 

4i 

4i 

0 
H 

31 

i 
1 

1 

2 

2 

4 

3.23 

0 

31 

3^- 

1 

1       4 

6 

69 
55 
23 
10 

6 

9 

IS 

17 
149 
173 

2 
22 
23 
19 

1 
14 

2 

5 

2 
1 

2 
1 

10 
1 

....|      59 
....     484 

3.46 
3.50 
3.49 
3.53 
3.31 
3.72 
3.69 
3.96 

.04 
.12 
.26 
.34 
.69 
.40 
.56 
.41 

3f 

1       5 

s 

44  '  144 

3| 

Vt 

1 
1  j    9     12 

2....     4 

1        9  i     ."? 

39 

18 

6 

15. 

469 

3| !    . 

1 

08 

fiO 

1 

23S 

64 

4 

34 

6 
13 
11 

2 

4| 

....1    2 

2 

14 

....       60 
. . . . '      22 

U 

1 
1 

1        7 

4f 

\ 

1 

8 

Totals  or  means..  3.75 

'        : 

1 

1 
f 

1    5 

13     25  1  55 

1 
170     119  {  469 

440 

88 

20  '     1     1408 

3.48 

.27 

36 


PIEBALD    RATS   AND    SELECTION. 


Table  9. — Classification  of  generation  9,  plus  series. 


Grade  of  parents. 

Grade  of  offspring. 

03 

0 

c 
0 

'to 

03 

bC 

If 

2 

2i 

21 

2f 

1 

3     31 

31 

3f 

4 

4i 

H 

31                

1 
6 
7 
2 

1 

1        1 

26       96 

63 

212 

177 

15 

4 

1 

4 

244 

591 

424 

45 

4 

3 

3.25 
3.43 
3.50 
3.65 
3  57 
3.57 
3.75 
3  50 

.25 
.19 
.25 
.22 
.43 
.55 
.50 
.87 

3|        

1 
2 

1 
4 

12 

19 

5 

2 

28 

10 

25 

64 

7 

1 

2 

1 

2 
8 

6 
1 

31          

1 

i 
45  1    50  !  224 

31              

25 

32 

105 
14 

4        

5    

4i                               

?, 

41              

1 

3 

41     

Totals  or  means,  3.78.  .  . . 

1 

1 

3 

5 

17 

38 

105 

110 

443 

473 

109 

11 

7 

1322 

3.54 

.24 

Table  10. — Classification  of  generation  10,  plus  series. 


Grade  of  parents. 

Grade  of  ofT.spring. 

Totals. 

Means. 

c 
_o 

'a- 

<n 

<u 
(-< 
fcO 

« 

0^ 

21 

2i 

2f 

3 

3i 

31 

Q3 

4 

^ 

A\    5 

3a 1  .. 

1 

1 
3 

9 
3 

13 

16 

4 

63 
95 
26 
11 
1 

117 

142 

61 

17 

4 

44 

62 

20 

7 

2 

8 

20 

7 

6 

1 

1  .... 

1 

5   .... 

2  .... 
1        2 

257 

347 

120 

44 

8 

3.69 
3.72 
3  76 
3.87 
3.84 

.06 
.15 
.24 
.25 
.41 

3| 

1 

4 

41 !:::: 

41 L--- 

Total  or  means,  3.88 

1 

1 

4     12     .^3 

196     Ml 

135 

4'> 

.1     , 

776 

3.73 

.15 

Table  11. — Classification  of  generation  11,  plus  series. 


Grade  of  parents. 


Grade  of  offspring. 


2f 


3^ 


3f 


4  Ul 


4f 


Totals. 


Means. 


Regres- 
sion. 


4|. 
4i. 


2 
87 
25 
16 

2 


7 

162 

87 

49 

13 

1 


2 

41 

65 

27 

5 

3 


Totalsor  means,  3.97. .    2 


13 


22  132 


319 


143  1  46 


12 


11 

333 

214 

110 

25 

4 


3.75 
3.70 
3.87 
3.81 
3.91 
3.94 


0 
.17 
.13 
.31 
.34 
.43 


697 


3.78 


.19 


TABLES. 


37 


Table  12. 

—Classification  of  general' 

'on  12,  plus 

series. 

Grade  of  parents. 

Grade  of  offspring. 

TO 

C 

d 
.2 

2§ 

3 

H 

31 

31 

4 

H 

4* 

41 

5 

5i 

3f 

4 
2 
10 
17 
7 
3 

20 

19 

53 

86 

19 

6 

1 

3 

7 
17 
59 
83 
29 
14 
4 
4 

4 

6 

12 

28 

11 

10 

1 

3 

35 

48 

145 

227 

71 

45 

7 

11 

1 

3.83 
3.96 
3.93 
3,91 
3.94 
4.13 
4.14 
3.90 
4.75 

-.08 
.09 
.07 
.21 
.31 
.24 
.36 
.72 
.25 

3| 

2 
5 
6 
3 

7 

2 

3 

1 
3 

4 

1 
1 

1 
2 

1 

1 

2 

U 

1 

4i 

4f 

1 
1 

1 

4* 

4f 

1 

5 

1 

Totals  or  means,  4.09 

1 

3 

3 

43 

207 

217 

75 

23 

11 

4 

3 

590 

3  94 

.17 

Table  13. — Classification  of  generation 

13, 

plus  selection  series. 

Grade  of  parents. 

Grade  of  offspring. 

Totals. 

Means. 

Regres- 
sion. 

2f 

3i 

31 

3i 

4 

4i 

4i 

4f 

4 

4 

17 
1 
5 

1 

1 

23 
8 

10 
5 

14 
3 

19 
16 
20 

3 
15 

1 

2 
5 

6 

1 
7 

65 
31 
43 
10 
40 
5 

3.67 
3.97 
3.93 
3.85 
3.98 
3.95 

.31 
.15 
.32 
.52 
.52 
.80 

4i 

.... 

1 

4J 

1 

41 

4^ 

.... 

4i - 

Totals  or  means,  4.22 

1 

4 

25 

63 

74 

21 

5 

1 

194 

3.88 

.34 

Table  14. — Summary  of  the  results  of  thirteen  generations  of  plus  selection  based  on  Tables  1-13. 


Gener- 
ation. 

No.  of 
offspring. 

j 

Moan     j     Mean 
parents,  j  offspring. 

I 

Standard 

deviation, 

parents. 

Standard 
deviation, 
offspring. 

Correlation, 
parents- 
offspring. 

Absolute 
regression 
of  offspring 
on  parents. 

Advrnce 

of 
parents. 

Advance 

of 
offspring. 

1... 

2... 

3... 

4... 

5... 

6... 

7... 

8... 

9... 
10... 
11... 
12... 
13... 

150 

471 

341 

444 

610 

861 

.      1,077 

.      1.408 

.'     1,322 

776 

.!        697 

.1        590 

194 

2.51 
2.52 
2.73 
3.09 
3.33 
3.52 
3.56 
3.75 
3.78 
3.88 
3.98 
4.09 
4.22 

2.05 
1.92 
2.51 
2.73 
2.90 
3.11 
3.20 
3. 43 
3.54 
3.73 
3.78 
3.94 
3.88 

.313 

.307 
.285 
.215 
.240 
.209 
.212 
.246 
.112 
.112 
.113 
.176 
,433 

.541 
.732 
.531 

.468 
.505 
.490 
.555 
.439 
.346 
.362 
.289 
.302 
.270 

.298 
.317 
.331 
.066 
.160 
.180 
.215 
.099 
.210 
.116 
.233 
.161 
.132 

.46 
.60 
.22 
.36 
.43 
.41 
.36 
.27 
.24 
.15 
.20 
.15 
.34 

.01 
.21 
.36 
.24 
.19 
.04 
.19 
.03 
.10 
.10 
.11 
.13 

-13 
,59 
.22 
.17 
.21 
.09 
.23 
.08 
,19 
.05 
,16 

-.08 

Total 

.        8,941 

1                     i 

i 

38 


PIEBALD    RATS   AND    SELECTION. 


Table  15.— Mean  grade  and  number  of  offspring  produced  by  parents  of  a  particular  grade  in  each 
generation  of  the  plus  selection  series,  based  on  Tables  1-13.  The  grade  of  the  parents  is  indicated 
at  the  head  of  each  column.  In  the  body  of  the  table  is  recorded  the  grade  of  the  offspring  {in  light- 
faced  figures)  and  the  number  of  offspring  {in  heavy-faced  figures) . 


Genera- 
tion. 

Grade  of  parents;  below,  grade  and  number  of  their  off.spring. 

Total 
number 
of  off- 
spring. 

2 

2\ 

91  1  2^ 

1 

2\  2f 

2f 

1 

21 

3  \ 

2.35 

12 

2.4?! 

31 

3i|3| 

31 

31 

31 

3|i4i 

1 

41 

4i  41 

4i  4f  4f 

1 

2 

3 

4 

5 

6 

1.76.... 
18.... 

1.701.28 
4C 

1.87.... 

20|.... 

1.87|l.92 

2.06 
37 

1.80 
133 

2.32 
54 

2.55 
11 

.... 

2.15 

5 

2.11 

44 
2.63 

64 

1       i 
2.12.... 

51 

1 

' 

150 

1.92* 
52 

2.46 
43 

■■■'1 

2.41 
23 

2.70 

2.50' 

1 

1  ■ 

471 

341 

444 

610 

861 

1,077 

1,408 

1.322 

776 

697 

590 

194 

2.06 
21 

.  .  .  . 

2.15 
17 

2.49: 

2.60 

143 

2.81 

15 

3.25 

2 

3.07 

2.69 
122 

2.81 
114 

3.172.91 
6      8 

1 

2.652.97 
is!    37 

3.002.87 
12     21 

2.892.703.02 
64    23    20 

3.08 

64 

3.16 

181 
3.23 

289 
3.50 

484 
3.43 

244 

2.75 

3.07 

14 

3.22 

78 

3.35 

184 

3.49 

469 

3.50 

591 

3.69 

257 

3.75 

11 

3.82 

35 

2.812.942.87 

3.35 

8 

3.26 

80 

3.49 

90 

3.53 

238 

3.65 

424 

3.72 

347 

3.70 

333 

3.96 

48 

3.36 

7 

3.41 

14 
3.53 

15 
3.31 

64 
3.57 

45 
3.76 

120 
3.87 

214 
3.93 

145 
3.67 

65 

3.00 
3 

2.843.102.963.10 
28  143  123  212 

2.873.092.973.18 

7 



3.72 

60 

3.57 

7 

3.87 

44 

3.81 

110 

3.91 

227 

3.97 

31 

3.75 

8 

28 

131    IRn    177 

3 

3.69 

3.46 

59 

3  25 

3.96 

9      . 

22      8 

3.75 

4 

3.84 

3.91 
25 

3.94 
71 

3.93 
43 

3.50 
3 

10 

4 

11  ... 

3.94 
4 

4.12 
45 

3.85 
10 

12  ... 

4.14 

3.98 
40 

3.90 
11 

.... 

.  .  .  . 

4.75 
1 

3.95 
5 

1 
13 

8,941 

Table  16. — Classification  of  the  offspring  in  generation  1  of  the  minus  selection  series. 


Grade  of  parents. 

Grade  of  offspring. 

Totals. 

Means. 

Regres- 
sion. 

+1 

0 

1 
—  * 

1 

3 

4 

1 

U 

n  If 

2 

-li 

1 

1 

2 

8 
1 
2 

3 
1 

3 
1 

1 
3 

2 
1 

1 
1 

1 

1 

8 

31 

6 

10 

134 

.86 
1.37 
1.05 

-.09 
.61 
.13 
.82 

If 

1 

3 

12 
1 

2 

u 

1| 

1 

1 

Totals  or  means,  1.46. . . 

2 

1 

4 

1 

15 

13 

4 

8      6 

2 

55 

1.00 

.46 

TABLES. 

Table  17. — Classification  of  generation  2,  minus  selection  series. 


39 


1 

1 

j         Grade  of  parents. 

1 

Grade  of  offspring. 

Totals. 

Mean.s. 

Regrcs- 
i    sion. 

+  1 

0 

i 

_1      1     1     3     1     1 

li    1^ 

li 

2 

3 

' 

2 
2 

2  j     1       6 
2  L...      2 

2      3 
2       S 

2 

3 
1 
4 

....!             19 

1            12 

1  17 

2  !          5 

1            57 

1.04 
1  05 
I.IS 
1.45 
1.11 
.67 
1.09 
1.10 

-.29 
i        .07 
.07 
-.08 
.39 
.95 
.66 
.77 

11 

li- 

1 

1    4 

6 

i. 

3 

1 
J. 

If 

1 

n 

1 

....lio 

....     1 

13 

1 

i< 

4 

.... 

If 

1 

3 

97 

If 

1 

1      2 

14 

3 
1 

1 
3 

1 

11 

Totals  or  means,  1.41 ..  . 

1     1 

1 

1      2      3 

....I          12 

2  ;    si    el    C  I  20     44 

1            '            I            :            > 

13     19 

14 

5          132 

1.07 

1        .34 

Table  18. — Classification  of  generation  3,  minus  series. 


Grade  of  parents. 

Grade  of  off.spring. 

Totals. 

Means.   ^,«.f  ^«- 
sion. 

0  -i 

1 

2 

3 

4 

1 

li 

11 

If 

2 

-n 

2  i    1  1    1 

!  1 

5                .85             .27 

20              1.05      '        .20 
28       1       1.03              .34 
28              1.31              .19 

48       1        1  22      j        .40 

03       1       1  26      :         49 

3       i       1  GO               87 

U 

1 
1 

3 
3 

1 
1 
2 

6     a 

1 

3 
1 

5 
9 

2 
4 
5 

8 

0 

1 
2 
5 

8 
7 

1 

3 

4 
8 

If 1.... 

4 
1 

10 
8 

11 
12 
10 
19 
3 

n !.... 

If i  1 

n ! . 

1 
1 

i| L. 

1 

: 

Totals  or  means,  1.56 

1       4 

1 

12 

30     61 

1 

19 

29 

23 

16          195       I       1.18              .38 

1 

Table  19. — Classification  of  generation  4,  minus  selection  series. 


Grade  of  parents. 

Grade  of  offspring. 

32 

O 

m 

Q 

§ 

C 

CO 

o 

+-I 

+i 

0 

1 
i 

i 

2 

f      1 

li 

li 

If 

2 

2i 

-li 

i 
i 

1 

1 

8 

1 

1  ;  1 

2  :  6 

16      8 

7      6 

22     11 

19      5 

1      2 

.  .  .  . 

1 

4 
29 
59 
40 
93 
95 

9 

1.56 
1.16 
1.31 
1.36 
1.34 
1.18 
1.36 

-.31 
.21 
.19 
.26 
.41 
.69 
.64 

If 

2 
3 
1 
4 
6 

3  i    4 

3       1 

li 

1 

1 

1        1 
21717 

6 

8 

If 

1 

....     4 

10 

19 

19 

2 

4 

11 

12 

3 

8 
16 

17 

If 

4 
1 

6 

n 

2 

1 

2 

2 

1 

Totals  or  means,  1.69 

2 

2 

3 

16 

10     32 

1 

66 

39 

51 

68    39 

1 

329 

1.28 

.41 

40 


PIEBALD    RATS   AND    SELECTION. 


Table  20. — Classification  oj  generation  5,  minus  series. 


Grade  of  parents. 

Grade  of  offspring. 

CO 

G 
03 

c 

o 
o 

0  -i 

1 

1 

1      U 

n 

If 

2 

2i 

2§ 

7 

2       1 

4 

5 

1 

6 

9 

12 

50 

11 

14 

3 

4 

: 

11 

5 

4 

24 

31 

25 

2 

?0 

1.09 
.99 
1.50 
1.25 
1.35 
1.30 
1.64 
1.52 
1.80 

-.22 
.01 

-.13 
.25 

.27 
.45 
.23 
.48 
.32 

1                              

2 
4 

1 
9 

4 
51 
53 

54 
202 
143 
109 

5 

If 

1 
3 

1 
14 

1 
9 
5 

29 
4 

11 

2 

10 

14 
7 
12 
55 
50 
28 
2 

2 

1 

u     

1 

11  i      7 

If 

4 

30 
12 

5 

15 

50 
29 
19 

1 

If 

1 
6 
4 

.... 

ll 

2 

1 

2 

21 

Totals  or  means,  1.73 

3 

18     21 

! 

63 

108 

64 

134 

172 

104 

13 

1     701 

1.41 

.32 

Table  2L — Classifix:ation  of  generation  6,  minus  series. 


Grade  of  parents. 

Grade  of  off.spring. 

CO 

o 

02 

o 

d 
o 

03 

0 

_1 

4 

h 

f 

1 

u  n 

If 

2 

2i  2h 

-11 

1 

1 

.... 

2 

1 

4 

16 

39 

61 

32 

4 

1 
4 
3 

20 

40 

22 

4 

1 

1 

4 

17 

39 

71 

37 

5 

2 

5 

4 

24 

94 

244 

502 

283 

85 

11 

.85 
1.44 
1.34 
1.45 
1.49 
1.59 
1.58 
1.52 
1.82 

.27 
-.07 
.16 
.17 
.26 
.28 
.42 
.60 
.68 

If 

6 
28 
77 
156 
89 
35 

6 

1 

2 
17 
43 
127 
76 
31 
3 

H 

1 
1 

8 
9 
5 

3 
11 
14 
24 
12 

2 

If 

1 

1 

1 
1 

3 

If 

12 
5 
4 
1 

2 
1 

1 

n 

2 

1 

2i 

2i 

1 

Totals  or  means,  1.86 

3 

6 

25 

66 

159 

94 

177 

396 

300 

22 

4 

1252 

1.56 

.30 

TABLES. 


41 


Table  22. — Classification  oj  generation  7,  minus  series. 


\ 

Grade  of  offspring. 

-I-' 
O 

^     i 

U3 

i 

§ 

.11 

10 

CO 

to 
Pi 

.07 
.08 
.22 
.28 
.38 
.32 
.74 
.62 
.09 
.88 

.28 

0 

i 

1 

h 

3 

4 

1 

U 

U  :  If  '   2     21   2i 

1                 1                j 

2f 

If 

1 

2 

4 

14 

3 

1 
6 

2        1 

5 

32 

330 

969 

260 

41 

4 

15 

18 

6 

! 

1.55 
1  67 
1.65 
1,72 
1.74 
1  93 
1.63 
1.88 
2.28 
1.87 

If 

1 

6 

8 

1 

35 
51 
14 

1 

20 
60 

11 

9 
106 
S';>4 

1 
1 

n 1  2 

2 

1 

47  1    99 
119     36S 

6 

22 

4 

6 

3 
3 

21 

1 

9, 

10       30  !    02     104 

2i 

i 

5       :0       19 

1    .... 

21 :::::::::::: 

2        2 

2i           .               1 

1 

'      4 

1         3 

10 
8 
3 

!"'"' 

2| 1  ...L...L.. 

5 

1 

21 

1 
1 

1 

3 

1 

Totals  or  means,  2.01 ....     2 

2 

17 

24 

101 

92 

211     594 

584  !  43 

8 

2 

1680 

1.73 

Table  23. — Classification  of  generation  8,  minus  series. 


Grade  of  parents. 

- 

Grade  of  offspring. 

0 

H 

to 

i 

d 
0 

03 
V 
U 
M 

m 

0     § 

1 

1 

U 

11         13 

1 

2 

2i 

2h 

21 

n 

1 
38 
12 

1 

1 

9 

550 

118 

19 

11 

8 

423 

187 

20 

17 

15 

1 

1 

21 

32 

3 

19 

1202 

396 

45 

30 

28 

6 

1.84 
1.81 
1.86 
1.87 
1.87 
1.93 
1.67 

.03 
.19 
.26 
.38 
.50 
.57 
.95 

2 

6 

8 
3 

22 

7 

1 

122 

31 

2 

1 

9 
2 

3 
3 

2i 

1 

2i 

2| 

2h 

3  i      6 
3  1      2 

4 

2| 

Totals  or  means,  2.05 

1 

6 

11 

52 

30     162  1  715 

671 

61 

11 

6 

1726 

1.80 

.25 

Tabi.^  24. — Classification  of  generation  9,  minus  series. 


Grade  of  parents. 

Grade  of  offspring. 

Totals. 

Means. 

Regres- 
sion. 

-h 

3 

4 

1 

H 

u 

If 

2       2J 

2^  2f 

2 . 

13 

1 

4 
1 

1 
1 

36 

268     420 

55 
40 
12 
12 
14 

811 
403 

1.90 
1  93 
1.93 

1.91 
2.07 
2.00 

.10 
.19 
.32 
.46 
.43 
.62 

2| 

25 

7 

110 
43 

218 

77 
89 

7 
6 

1 

2i 

1 

1 

148 

2f 

1 

1 

7      61 

i 

4!....         175 

5      2           53 

....!....            1 

i 

2h 

2 

6       23 

2f 

1    

1     Totals  or  means,  2.11. . 

1 

l|l6 

7  j  77  1  488     828  '  133 

!       ! 

32       8  ;      1,591 

1.92 

.19 

42 


PIEBALD    RATS   AND    SELECTION. 

Table  25. — Classification  of  generation  10,  minus  series. 


Grade  of  parents. 

Grade  of  offspring. 

Totals. 

Means. 

Regre.s- 
sion. 

-1 

U 

n 

If 

2 

2i 

2h 

2f 

3    31 

1 

2 

1 
2 

2 
1 

13 

15 

5 

4 

120 

100 

45 

16 

9 

5 

8 

2 

287 
251 
92 
60 
27 
17 
10 
3 

43 

78 
58 

7 
22 
13 

2 
4 
4 
3 
2 

2 

1 

1 

473 

474 

217 

119 

67 

49 

27 

20 

1.96 
2.00 
2.05 
2  05 
2.13 
2.15 
1.95 
2.19 

.04 
.12 
.20 
.32 
.37 
.47 
.80 
.68 

2i 

2i 

2i           

.  .  .  . 

1 

29  I    6 

, 

2i 

19 

17 

6 

6 

10 
8 
1 
5 

21 

1 

1 
1 

1 

. .. . 

21                       

1 
1 

2i      

2 

Totals  or  means,  2.18. . 

3 

6    40 

I 

305 

747 

256 

72     17 

3 

1 
2         1,451 

2  01 

.17 

Tabijj  26. — Classification  of  generation  11,  minus  series. 


Grade  of  parents. 

Grade  of  offspring. 

o 
H 

a 

1 

CO 

M 

o 

-1 

U 

n 

If     2     2i 

i 

21 

2| 

3 

3§ 

2      

7 

20 

49 

24 

i 

3 

4 

2 

26 

75 

111 

112 

32 

18 

9 

2 

14 
45 
76 
66 
21 
15 
4 
1 

3 

26 
50 
33 
16 
11 

3 

12 

13 

20 

7 

4 

1 

1 

6 
4 

51 

2.08 
2.15 
2  13 
2.16 
2  21 
2.20 
2.23 
1.95 

.08 
-.03 
.12 
.21 
.29 
.42 
49 
.92 

2i 

4 

12 

4 

1 

1 

1 

183 
318 
?fi8 

21 

1 

2f 

2      1 

2 

2h 

81 

52 

?6 

21 

1 

21 

1 

2 

1 

2| 

5 

Totals  or  means,  2.30 

1 

3 

2 

22 

113 

385 

242     142 

57 

12 

5 

1 
1     084 

2.15 

.15 

Table  27.- 

■Classification  of  generation  12, 

minus  series 

• 

Grade  of  parents. 

Grade  of  offspring. 

Totals. 

Means. 

Regres- 
sion. 

1 

n 

If 

2 

2| 

21 

2f 

3 

3i 

3^ 

2 

2 

6 

16 

21 

12 

5 

7 

65 

67 

116 

81 

38 

8 

7 

5 

1 

26 

45 

64 

75 

36 

7 

9 

5 

10 

1.98 
2.14 
2.15 
2.11 
2.24 
2  32 
2.45 
2.51 
2.65 

.02 
-.02 
.10 
.26 
.20 
.30 
.30 
.36 
.35 

2i 

1 

2 
1 

1 

15 

29 
24 
53 
37 
14 
14 
5 

4 

6 
3 
17 
12 
10 
15 
16 

1 

118 
166 
230 

2i 

1 

2f 

1 

3 

8 
3 
7 
5 

2| 

242 

137 

42 

53 

21 

1  .... 

2f 

2i 

1 

3 

2       1 

35 

Totals  or  means,  2.44. . 

1 

5     63 

394 

268 

191 

83 

27 

1 
3      2 

1.037 

2.23 

.21 

TABLES. 


43 


Table  28. — Classification  of  generation  13,  minus  series. 


Grade  of  offspring. 

Grade  of  parents. 

Totals. 

Means. 

Regres- 
sion. 

If 

2 

2i 

21 

2f 

3    31  3| 

2J 

3 

30 

12 

17 

4 

1 

.,..'.... 

67 

2.22 

.03 

21 

4 

40 

50 

46 

24 

5 

iL... 

170 

2.35 

.02 

21 

4 

32 

34 

47 

26 

8 

2j    1 

154 

2.40 

.10 

2f 

2 

11  j    21 

41 

28 

3 

2  |.... 

108 

2.47 

.15 

21 

1 

5 

2 

9 

7 

6 

1    .... 

31 

2.56 

.19 

21 

4 

7 

8 
5 

6 
5 

1 
1 

i 

26 
15 

2  43 
2  50 

.44 
.50 

3 

3  i      1 

Totals  or  means,  2.50 

14 

125     127 

173 

100 

25      6       1 

571 

2.39 

.11 

..... 

Table  29. — Summary  of  the  results  of  thirteen  generations  of  minus  selection,  based  on 

Tables  16-28. 


Genera- 
tion. 

i               1 

No.  of 
offspring. 

Mean, 
parents. 

Mean, 
offspring. 

Standard 

deviation, 

parents. 

Standard 
deviation, 
offspring. 

Correla- 
tion, 
parents- 
offspring. 

Absolute 

regression 

of  offspring 

on  parents. 

Advance 

of 
parents. 

Advance 

of 
offspring. 

1... 
2... 

55 
132 

1.45 
1.41 

1.00 
1.07 

.208 
.342 

.515 
.493 

.46 
.34 

-.033 

-.05 

.07 

3... 

195 

1.56 

1.18 

.196 

.484 

.206 

.38 

.15 

.11 

4... 

329 

1.69 

1.28 

.190 

.460 

.020 

.41 

.13 

.10 

5.  .  . 

701 

1.73 

1.41 

.233 

.500 

.184 

.32 

.04 

.13 

6... 

1,252 

1.86 

1.56 

.185 

.438 

.164 

.30 

.13 

.15 

7... 

1,680 

2.01 

1.73 

.132 

.352 

.143 

.28 

.15 

.17 

8... 

1,726 

2.05 

1.80 

.107 

.283 

.094 

.23 

.04 

.07 

9... 

1,591 

2.11 

1.92 

.184 

.285 

.059 

.19 

.06 

.12 

10... 

1,451 

2.18 

2.01 

.255 

.242 

.158 

.17 

.07 

.09 

11... 

984 

2.30 

2.15 

.229 

.349 

.081 

.15 

.12 

.14 

12... 

1,037 

2.44 

2.23 

.310 

.372 

.406 

.07 

.14 

.08 

13... 
Total 

571 

2.50 

2.39 

.177 

.317 

.235 

•" 

.06 

.16 

11,704 

!             i i 1 

■      1                i                i                1              : 

44 


PIEBALD    RATS   AND    SELECTION. 


Table  30.— Mean  grade  and  number  of  offspring  produced  by  parents  of  a  particular  grade  in 
each  generation  of  the  minus  selection  series,  based  on  Tables  16-28.  The  grade  of  the 
parents  is  indicated  at  the  head  of  each  column.  In  the  body  of  the  table  is  recorded  the 
mean  grade  of  the  offspring  {in  light-faced  figures)  and  the  number  of  offspring  {in  heavy- 
faced  figures) . 


Generation. 

Grade  of  parents;  below,  grade  and  number  of  their  offspring. 

Total 
number 
of  off- 
spring. 

li 

1^ 

U 

If 

11 

11 

2     2| 

2i 

2|     21 

2f    2i 

2|     3 

1 

1.34 

8 

1.17 

1.05 
20 

.85 

31 

1.45 

5 

1.04 

9R 

1.37 

6 

1.11 

37 
1.31 

28 
1.31 

59 
1.25 

53 
1  34 

1.05 

in 

" 

55 

132 

195 

329 

701 

1.252 

1,680 

1,726 

1.591 

1.451 

984 

1.037 

£71 

2 

0.67 

3 

1.22 

48 
1.36 

40 
1.35 

54 
1.46 

94 

1.55 

5 

1.09 

27 
1.26 

63 
1.34 

93 

1.30 

262 

1.49 

244 

1.67 

32 

1.10 

i 

3 

12 

1.96 

I 

4 

3 

1.18 

1.561  1.16 

1.36 

5 

4 

29 

1.50 
51 

95         9 
1.64   1.52 

1.80: 



6 

143     109        5 

1.59,  1.58  1.52 

502    283      85 

1.65   1.72   1.74 

1.82      ; 

7 

24 

11 

1.88 

1.93 

1.62 

2.28'  1.87 

8 

330     969     260      41 

4       15       18 

6 

1.84  1.81   1.86 

1.87 

36 

1  93 

1.87 

30 

1.91 

1.92   1.67 
17        6 

2.07 

9 

19   1170 

377 

1  93 

1.90 

10 

811     403     148 

1.96;  2.00  2.04 

478     474     217 

2.081  2.15  2.13 

175       53 

1 

2.05 

2  13 

\"" 

2.15  1.95 

2.18 
20 

1.95 
5 

2.51 
53 

2.43 
26 

2.65 
35 

2.50 
15 

11 

119      67      49      27 
2.16  2.21    2.20  2.26 

12 

51,    183     313 
I.QSJ  2.14  2.15 

268      81 
2.11  2.24 

52      26 
2.32  2.45 

13        

10     118 

166 

230     242     137       42 

2. .35   2.40   2.47  2.55 

\ 

67 

170 

154 

108      31 

11,704 

Table  31. — Results  of  a  first  return  selection  from  generation  6,  minus  series. 


Grade  of  parents. 

Grade  of  offspring. 

Totals. 

Means. 

Regres- 
sion. 

0 

1 

-f-i-U-ii-if-2 

i     1      '      :      ! 

_| 

2 

1 

4 

1 

3       4    j     8   ]     8    ...           26 
10  >    14       17    i    20    i    4  i         7fi 

1.08 
1.33 
1.30 
1.41 

-.60 
-.71 
-.65 
-.63 

-1 

2 

3 

'                    1 
1         ...1      2    1      1 

5 
2           11 

-1 

2 

2 

1        4 

Totals  or  means,  .GO 

2 

4 

6      5     16  {    18 

1 
28       33        6 

118 

1.28 

-.68 

TABLES.  45 

Table  32. — Results  of  a  second  return  selection  from  generation  6,  minus  series. 


Grade  of  parents. 

Grade  of  offspring. 

Totals. 

Means. 

Regres- 

0 

1 

2 

_3 

-1 

-n-n 

sion. 

1 

1 

3 

4 

5           2           4 

19                0.";      I          AK 

"    1 

Table  33. — Results  of  a  third  return  selection  from  generation  6,  minus  series. 


Grade  of  parents. 

Grade  of  offspring. 

Totals. 

Means. 

i 

Regres- 
sion. 

0 

-i 

1 

—  '2 

1 

-f-l-lij-U 

-lf-2 

1 

5 
?, 

2 
3 

2      3 

5      4 

....     1 

1 

2 

. . . .  1 
3 

13 
21 
13 
31 

.63 

86 

1.61 

1  35 

j      -.38 

'      -.49 

-.49 

1      -.10 

3 

7 

-1| 

2 
9 

3 

8 

4 
5 

3 
2 

-U     

1 

2      4 

Totals  or  means,  .83 ... . 

2       7 

C 

1 

0   ;    12        11          14 

1 
12        5  i         78 

1.14 

1      -.31 

Table  34. — Results  of  a  fourth  return  selection  from  generation  6,  minus  series. 


Grade  of  parents. 

Grade  of  offspring. 

-2 

Totals. 

Means. 

Regres- 
sion. 

0 

_i 

4 

_i 

2 

3 
4 

-1 

-u 

-H 

-If 

1 

1 
1    1 

2 
1 
1 

2 

1 
1 
1 

1 

2 

2 
2 

4 

2 

3        2 

16 
5 
3 

10 
10 

1-34 

.70 

.83 

.83 

1.17 

1.42 

-1.09 

-  .20 

-  .21 

-  .08 

-  .30 

-  .42 

-1 

1 

5 

1 
2 
2 
1 

3 

1 

1 
2 
3 

-I 

3 

1 
1 

-1 

Totals  or  means,  .63 

3  1    2       6      4 

7 

10 

8 

6       4           50 

1.17          -    54 

[ 

Table  35. — Results  of  a  fifth  return  selection  from  generation  6,  minus  series. 


Grade  of  parents. 

Grade  of  offspring. 

-H 

Totals. 

Means. 

Regres- 
sion. 

0 

4 

1 

—  2 

3 
■~4 

-n 

-1 

2 

2 

3 

4 
4 
5 

1.00 

1.12 

.25 

-.75 
-.37 
+.62 

_3 

1 

7 

—  -o 

2 

2 

1 

Totals  or  means,  .65 

3          2 

, 

2 

2 

3 

13 

.75 

-.10 

46  PIEBALD    RATS   AND    SELECTION. 

Table  36. — Results  of  a  sixth  return  selection  from  generation  6,  minus  series. 


Grade  of  parents. 

Grade  of  offspring. 

1  . 

1  " 

tA 

. 
c 

(0 

+U 

+  1 

+f 

\ 

0 

1 

-H 

-u 

-n^i\ 

Mean 
Regrc 

-1 

1 

1 

3 

1 

2 
4 

1 

1 

....|    2 
1  i    4 

2 

!H 

...      '13 

.29 
.35 
.87 
.25 

-.17 
-.10 
-.50 
+.25 

_i 

2 

1 

-f J  .. 

....i    2\    1 
1       2  1     1 

1 

1 

1        6  : 
1 
s 

-i 

1 

I 

Totals  or  means,  .26 . . 

i 

1 

1 

1  1    2  1    1  j  4  1    7 

2       5 

8 

2 

1 

1       36  i 

1 

.39 

-.13 

Table  37. — Results  of  a  return  selection  from  generation  7,  minus  series. 


Grade  of  parents. 

Grade  of  offspring. 

Totals. 

Means. 

Regres- 
sion. 

2 

3 

-1    -\\ 

-H  -If 

-2 

-1 

1 

2 

3 
2 

3 

7    1      8 

3 

25 

1 
t 

-1 

2 

( 
1      2 

!       8 

Totals  or  means,  .78 

1 

1 

2         3    !      5         3         7        10 

3             33       1       1.15            -.37 

Table  38. — Results  of  a  return  selection  from  generation  8,  7ninus  series. 


Grade  of  parents. 

Grade  of  offspring. 

1 

Means. 

Regres- 
sion. 

-f   -1  -U  -H 

1 

1 

i 

1  lotals. 

-2| 

-1 

1 

3 

9 

3    1         13 
3    i         13 
2    ■           2 
2    1         13 

1.69 
1.21 
2.00 
1.56 

-1.19 

-  .59 

-  .13 

-  .56 

-f 

1 

7 

-1 

1 

2 

4          4    i 

Totals  or  means,  .72 

■i  ^ 

2 

7 

1 
19 

10             41 

1.51 

-  .79 

TABLES.  47 

Table  39. — Results  of  a  return  selection  from  generation  11,  minus  series. 


Grade  of  parents. 

Grade  of  offspring. 

T^^+r.lrt 

T^T. 

Reeres- 

1 

4 

-If 

-2 

-2i 

-21 

lOtalS.    ivieaiis.         •" 

1                     sion. 

-If 

1 

3           7 

3 

2 

16          !          1   OS               _  35 

1 

Table  40. — Results  of  a  return'jelection  from' generation  6,  'plus  series. 


Grade  of  parents. 

Grade  of  offspring. 

Totals. 

Means. 

Regres- 

+1+1+2 

2i    2|    2f 

3 

31  31 

sion. 

2.. 

I      i            1      i 

1113       3  12  12 

3 

11             17       1       2  36            -  HB 

■ 

1 

Table  41. — Results  of  a  return  selection  from  generation  11,  plus  series. 


Grade  of  parents. 

Grade  of  offspring. 

Totals. 

1 

Means. 

1 

Regres- 
sion. 

21 

2§ 

21 

3    31 

3i 

31 

4 

4i 

2f 

2f 

3 

1 

2       3 

9     a  [   a 

1 

8 

1 

22 

17 

9 

5 

2       4       1 

1   .             L.. 

2 
4 
3 

4       1 
1    .... 

'^ i  ••■ 

3} ! 

.■'  1 '....!.. .. 

Totals  or  means,  2.79 

2 

1     '     1 

1        t 

3 

1     1 

4  1  10      6     12 

1               1 

10 

5|    1 

53 

3.32 

-.53 

48 


PIEBALD    RATS   AND    SELECTION. 


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TABLES. 


49 


Table  44. — F3  generation  produced  by  extracted  hooded  rats  (F2  generation)  derived  from  a 
cross  of  a  hooded  rat  of  the  minus  series  with  a  wild  rat.     {See  Table  42.) 


Grade  of  (F2)  parents. 

Grade  of  (F3)  offspring. 

Means. 

d 
'to 

CO 

tt 

Ph 

.71 
.44 

+1 

3 

4 

1 

U 

n 

If 

2  2i  21  2f 

3 

3J 

3^ 

f 

2       1 !    4      3  14 

...J....' 1 

1 

1 

2 
1 

1 
3 

I 

1 

1  ... 
1    1 

19 
34 

+  .04 
+2.06 

2* 

2     9. 

1    al  a  .  R 

■■■,■■■ 

4   i   3 

1 

1     i 

Table  45. — Ft  generation  from  minus  series  crossed  with  wild. 


Grade  of  (Fs)  parents. 

Grade  of  (F4)  offspring. 

CO 

r^ 

0 

'co 
M 

1 

0 

+i 

1 

3 

4 

1 

n 

1^ 

n 

2 

21  2§  2f 

3 

31  3^ 

'3 
0 
H 

2i 

2 

1 

2 

1 

1 

1 
2 
1 

3 
1 

1 

6 
6 

1 

3 

6 
3 

1 

9 
8 
1 
3 

1 

16 
3 
1 
5 
1 

25 
9 
1 
9 
3 

14 

10 
4 

2 

4 

1 
1 

96 
44 
8 
39 
18 

1.78     .47 

1.67     .70 

91  1  nn 

2f 

— 

1 

.... 

2| 

1 

3 

■ 
5 

1 

7 
2 

3 
5 

2.02 
9.  R9. 

.98 
.30 

3| 

Totals  or  means,  2.52. 

1 

1 

1 

3 

3 

1 

5 

5 

16 

10 

22 

26 

47 

20 

23 

14 

7  I    1  i    2  ;  205;  1.86     .66 

1       1       1       1 

Table  46. — Fs  generation  from  minus  series  crossed  tviih  wild. 


Grade  of  (F^)  parents. 

Grade  of  (Fj)  offspring. 

0 

CO 

d 
0 

CO 

0 

0 

+h 

+1 

1  n  n 

If 

2 

2i 

2h 

2f 

3 

3J 

2 

2i 

] 

1 

3 

1 

3 

2 
1 
1 

12 

1 
3 

1 

15 
4 
4 

2 

10 

1 
1 

3 

2 

— 

2 

2 

3 
4 

2 
4 

1 

1 

53 
13 
13 
14 
7 
5 
14 

2\ 

^*' 

.... 

9 

. . . . 

2f 

1 

2 

1 

2 

1 

21- 

2 

1 

1 

1 

1 

7 

3     . 

1 

^^•■1  ' 

4 

_ 

Totals  or  means,  2.27 

2 

1 

3 

6 

3 

6 

17 

28 

16 

8     21 

1 

7 

1 

119 

2,06 

21 

50 


PIEBALD    RATS   AND    SELECTION. 


Table  47. — F^  generation  from  minus  series  crossed  with  wild. 


Grade  of  (Fs)  parents. 

Grade  of  (Fe)  offspring. 

-(J 

o 

.2 

o 

;-! 

P5 

+1 

i    1 

li  14 

If    2 

2J 

21 

1 

2i!  3 

?'l 

34 

3i 

2' 

1    1 

1 

3 

1 

4 
2 
2 

4 

1 

8 
2 
1 
2 

1 

13 
4 
2 
5 
4 
4 

4      4 
3      3 
3!.... 

7 
5 
3 

8 

3 
4 
4 
2 

2 

48 
26 
20 
31 
40 
29 

2.14 

2.24 
2.26 
2.35 
2.74 
2.73 

.23 
.26 
.36 
.40 
.13 
.39 

•^8 

2i                            

21 

1, 
1 

! 

1 
5 
5 
2 

1 

.... 

* 
21                       

1 
2 

3 
9 
4 

2f              

7      6 

2 
1 

2 

3i               

Totals  or  means,  2.G9. . 

i   1 

2 

1       2 

6     13 

14 

32     13  i  23 

1 

1 
37  1  30 

1 

15 

4 

2 

194 

2.41 

.28 

Table  48. — F^  generation  from  minics  series  crossed  with  wild. 


Grade  of  (F7)  ofTspring. 


Grade  of  (Fc)  parents. 


!+f 


21 . 
21 . 


3L 


+1  14 


1^ 


0     91  i  91    93  I  q     qi  I  01 


3f 


m 

-t-> 
o 


—  I 


12 

2 

9 

6 

5  \ 

4 

1 

2; 

3 

1 

3 

4 

3 

3 

Ij 

3 

1 

2 

12  !    1 

3L.. 

3 ;  1 

5 1  2 

13 
2      2 


53 
28 
10 
12 
27 
14 
10 


1    o  ! 


Totals  or  means,  2.80 3     2  '   8  j  9  .  26    12  j  28 


23  I  26  ^  10  I    6 


1     154 


a 


o 


2.41 
2.14 
2.32 
2.93 
2.59 
2  63 
2.85 


.21 
.61 
.55 
.07 
.53 
.62 
.52 


2.46     .34 


Table  49. — F^  generation  from  minus  series  crossed  with  wild. 


Grade  of  (F7)  parents. 

Grade  ot  (Fs)  offspring. 

Means. 

Regres- 
sion. 

+1|  2 

1 

2i  2||2f 

1 

3 

31 

34 

AUtcli6. 

31 

2| 

1 

....    1 

i         2 

2  75 
2.74 
2.60 

0 
.01 
.52 

3 

11     3 

5 

1 

1            24 
19 

3i 

12       3       3       5       3 

Totals  or  means,  3  08 

2       3      6      9     13      4      6       1 

1            45 

5.07 

.41 

TABLES. 


51 


so 

v> 
S 


CO 


fa- 


c 

CO 

CO 

o 
J- 
o 


0^ 


O 


•UOIC^■BlA^p 

pj'bpu's^g 

t-;              CO      CO      <3>      O      O      C-       O      0>      OO      O 

1.01 
.60 

to 
0 

•SUT38I,\[ 

§        Lo   S   ^   S   2   g   !3   8   S   ^  ^ 
+        1     I+  +  +  +  +  4-  +  + 

rM                           O              ^^      O      Ol      CO      <D 
C»»                        O             CO      -^      00      o      »o 

+         +     +  +  +  +  + 

-t- 

•sjB^ox 

CO           coccoo»-Hooooeococo« 

«i»                           HJt              CO      CO      t*      CI      «0 
O                              «                            rH                  CT       M 

CO 

CO 
+ 

TH 

- 

raw 
+ 

1-^ 

rH 

rH           : 

Cv) 

+ 

>       1-4 

•      <D 

l>- 

1-1 

- 

+ 

C<l 

■     CO 

lO                              rH 

rH 

1— t 

C>1 

+ 

TH 

.        t-1 

^.        (M        rH        rH 

t~                                  '. 

rH 

i      - 

CI 

+ 

:      IH       rH       CO       _ 

;  CO  rH  CO     : 

CO                              wt 
rH 

rH      •* 

•.-5 

+ 

- 

.H      W      CO      CO 

rH        •*        M        •^h            : 

O                              r. 

rH      ^ 

rH 

CO 

to 
a 

1— ( 

+ 

CO 

iH        rH 

.—1 

rH        T}< 

iM     : 

S                         <N 

^     rr 

»o 

Grade  of  offspri 

1— t 

+ 

<o 

ro    M    5D    .H    e^    u5 

■* 

S                           '»-              rH 

rH     e«     CJ 

® 

+ 

t-1 

rHCOO<»H?5ir5COCO          • 

Cd     -^i    CO 

o> 

+ 

00                tHt-(i-ItHC^i-H           •00»-llO           • 

■*                              rH                rH       CO 

CO     10 

<M 

+ 

C^l                »-(           -f-ti-li-l           .*M<i34           '^H           I 

O                              rH 

CO 

rH        ■* 

QO 

o 
I 

OS                                  • 

CO        «        W       rH 

00 
CO 

1— t 

O                              rH                    1 

rH 

rH       rH 

1 

^ 

N 

rH      rH 

■* 

1 

00                  •CO'Tl«C0CO*-<C^Jt^»-t 

S 

r-t 

1 

1-1              ■^      1-1      *-*      CO      !>•      T-» 

u5     : 

a 

C4 

M 

1 

M 

M 

^H        C^        tH 

rH 

t>- 

rt|C« 

1— 1 

1 

; 

i-H        1-1        T-*        Cq        T-4 

rH       rH 

rH 

Cb 

- 

I— 1 
1 

N        1-t 

rH    eq 

rH 

1 

; 

rH 

t-H 

J 

m 

o 

o 

6 

Series  1,  Fj  generation;  9  9  —2,  gen.  6; 
d'c?  +34  or  31,  sen.  5 

T-H 

1 

a 

0) 

rH 

tn 

o: 

'tH 

ID 

CO 

: 

Hie 
1 

• 

d 

Hioe 
+ 

+ 

+ 

+ 

•      rH|0C 
1-H     rH 

+    + 

1-H 
+ 

r<:N 

rH 
+ 

Series  1,  totals  or  means,  F2  generation. . . 

Series  2,  F]  generation;   9  9    +3|,  gen. 
10;  cf  -3L  gen.  10 

0" 
.2 

'■*3 

(H 

oQ 
Ph 

ci 

m 

<D 

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02 

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+ 

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+ 

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+ 

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1-1 

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0 

c 

c 

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6 

S-. 

0 
re 

0 
•t-> 

(N 

CD 

IV 

52 


PIEBALD    RATS   AND    SELECTION. 


Table  51. — Fi  offspring  of  the  original  "tnutanl"  viale,  4763,-^6^,  mated  with  females  of  the 
plus  series,  and  with  the  "mutant^'  feynale,  5153,-\-5\. 


Grade  of  mother. 

Grade  of  offspring. 

Lov?er  group. 

Upper  group. 

+3 

3i 

31 

31 

4 

4i 

41 

o 

Standard 
deviation. 

5 

5i 

5§ 

5f 

6 

Standard 
deviation. 

+31 

ii 

4 
3 
4 

11 
7 
9 
3 
1 

1 
3 
4 

1 

1 

.... 

18 

13 

20 

5 

2 

3.69 
3.75 
3.78 
3.90 
3.62 

1 

.    ..     1 

3 
4 

7 
2 

9  !    4 

5  1    ^ 

. 

18 
13 

18 

7 

5.51 
5.44 

5.40 
5  46 

31        

4 

4i 

....    1 

1 
1 

1 

2 

5 
4 

4 
1 

A\ 

1 



Totals  or  means.  .  .  . 
Mutant  9  5153, +5|. 

1 

1 

12     31 

9 

3       1 

58 

3.73 

.24 

4     16 

23 

12 

1 

56 

5.45       .23 

3 

3 

6 

3.87 

6 

4! 

10 

1 
5  60  ' 

1 

Table  52. — Fi  offspring  of  the  original  mutant  male,  4763, -\-5\,  viated  with  females 

of  the  minus  series. 


Grade  of 

Grade  of  offspring,  lower  group. 

mother. 

_i  _a_i_ii  n  '4-1 

+\ 

+1- 

fl 

+u 

+U 

+lf- 

f  2J  Totals. 

i              i 

INIeans. 

Standard 
deviation. 

-2 1 

1 

! 

1  .... 
....■    1 

3 
1 
2 
1 

1 
2 

2 

1 
2 

1    . 

2 

1 

1 
1 

1 
1    . 

1            13        1 

+  .40 
+  .70 
+  .31 
+  .18 

-2\ 

1 

2 

3 

-2h 

1 

4 

4 

-2| 

....     1 

Totals  or  means 

■ 

2 

1 

l|    2 

7 

3 

5 

4 

2 

3 

2 

2  j   1        35 

+  .49 

.77 

Grade  of  mother. 

Grade  of  offspring,  upper  group. 

1        ( 
+4+4i+4^l+4^ 

!            i 

+5 

Totals. 

Means. 

Standard 
deviation. 

-2 

•     2 

e     1              1      9 

2 

12 

9 
6 

4 

4.46 
4.53 
4.21 
4  19 

-21 

2 

1      Q 

-2\ 

>> 

1 

! 

-2| 

0 



\"" 



Totals  or  meant 

3 

'      A    !      1*      '       *      '       ' 

31 

;                   1 

■l 

i  " 

1  " 

!  " 

1    2 

4.43              "'        ' 

iV 

TABLES. 


53 


Table  53. — Classification  of  the  descendants  through  three  generations  of  the  two  original 
mutants,  &J!i763,-\-5^,  and  9  5153, -]r5\.     The  -parents  are  in  every  case  of  grade  6^  or  5j. 


Generation. 

Grade  of 

offspring. 

Lower  group. 

Upp 

er  group. 

3i 

3^ 

3f 

4 

O 

CO 

Standard 
deviation. 

5 

5i 

5^ 

O 

1 

a 

Standard 
deviation. 

Fi 

3 

3 

6 
2 

3.87 
3.37 

6 

15 

7 

1 
4  1  10 

10  1  30 

3     11 

5.60 

Fo 

1 

1 

2 

3 

1 

5.52    

5.55    

F, 

Totals  or  means 

1 

1 

3 

3 

8 

3.75 

.25 

2 

4 

28 

17     51 

5.54       .19 

Table  54. — F2  descendants  of  the  original  mutant  male,  4763, +5^,  mated  with  females  of  the 

phis  series.     {Compare  Table  51.) 


Grade  of  (F2)  offspring. 

Grade  of  Fi  parents. 

Lower  group. 

Upper  group. 

3 

3i 

3^ 

31 

4 

4i 

41 

m 
-a 
0 

H 

<u 

u  0 

02  T) 

5 

H 

H 

H 

03 

CO 

Standard 
deviation. 

Lower  group 
parents . . . 

'3|.... 

03 

2      ' 

10 
2 
2 

8 
2 
6 

1 
7 
8 

22 
15 
22 

3.56 
3.92 
3.92 

1 

2 
4 

1 

1 

31 

1 

2 

49 
5 

13 

2 

5.50 

Totals  or  mea 

Upper  group 
parents. . . 

ns .  . . . 

2 

3 

14 
1 

16 

16 

6 

2 

59 

1 

10 

3.78 
3  50 

.33 

51.... 

1 

1 

3 

1 

4 

.... 

3.90 

2 

6 
2 

70  j  5.51  1 

51.... 

! 

7  I  5.43 

ms . . . . 

1 

'■ 

Totals  or  me? 

....     li    2 
i 

3       1 

1 

4    ....    11 

1 

1 

3  86       .35 

i 

2 

8 

56 

13 

79    5.50 

.15 

54 


PIEBALD    RATS   AND    SELECTION. 


01 

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Hoo  rtl'*  c5|<r  Hc«  i.*:qc 

North  Carolina  State  Library 


EXPLANATION  OF  PLATES. 


Plate  1. 


Top  row,  a  set  of  arbitrary  grades  used  in  the  classification  of  the  rats  studied. 

Middle  and  bottom  rows,  skins  of  rats  graded  as  indicated  by  the  numerals  above  each 

skin.    The  animals  graded,  +4,  +4|  and  +4f ,  being  entirely  dark  above,  are  shown 

in  ventral  view. 

Plate  2. 

cf  8000,  a  wild  gray  rat  caught  in  Cambridge,  Mass.,  October,  1908. 

96176,  a  black  hooded  rat  of  grade  —If  which  was  mated  with  cfSOOO. 

(^8021  and  6^8018,  Fi  offspring  of  the  pair  just  described,  cfSOOO  and   9  6176.     c:f^8021  is 

of  grade  +4|,  with  considerable  white  below.     Notice  also  liis  white  legs  and  compare 

with  those  of  his  father.     c?8018  is  of  grade  +5^.     Notice  white  areas  on  belly  and 

front  legs. 
8075-8078,  four  Fo  progeny  (grandchildren)  of  cfSOOO  and  96176.     8075  is  black  hooded, 

grade  -f ;  8076  is  gray,  +4 J;  8077  is  gray,  +4;  8078  is  black,  +5^. 

Plate  3. 

cflOS,  an  evenly  marked  rat  of  grade  +3.     A  female  rat  of  this  same  grade  was  mated 

with   c?8000  (the  wild  gray  male).     Two  litters  of  F2  grandchildren  are  shown  in 

8062-8067,  and  8070-8074. 
8062  is  black  hooded,  +2;  8064  is  gray  hooded,  +2;  8065  is  black,  +5;  8066  is  gray, 

+5j;  8067  is  gray,  +4:^  (notice  white  feet). 
8070  is  black  hooded,  +2;  8071  is  gray  hooded,  +lf;  8072  is  gray,  +5;  8073  is  black, 

+5§;  8074  is  gray,  +5f. 

56 


PLATE  1 


—  1 


+  1 


+  3 


+  4- 


PLATE  2 


$   8000 


ri'^. 


J"    ,        /^'^ 


$  6176 


i^ 


5  8021  F^ 


8078 


2 


QQll 


Jt 


5  801  8  F, 


J^.,     ^  gg||^_^ 


8076 


8075 


A( 


PLATE  3 


3    103  -f  3 


806'7  8066 


8073  8074- 


V\ 


V 


pp^^ 


.•n«  state  university  Libraries 
»4orthCaro\.na  State 


